ANRV345-BI77-11 ARI 28 April 2008 12:17

Mechanism of EukaryoticHomologousRecombinationJoseph San Filippo,1 Patrick Sung,1

and Hannah Klein21Department of Molecular Biophysics and Biochemistry, Yale University Schoolof Medicine, New Haven, Connecticut 06520; email: patrick.sung@yale.edu2Department of Biochemistry and Kaplan Comprehensive Cancer Institute,New York University School of Medicine, New York, New York 10016;email: Hannah.Klein@med.nyu.edu

Annu. Rev. Biochem. 2008. 77:22957

First published online as a Review in Advance onFebruary 14, 2008

The Annual Review of Biochemistry is online atbiochem.annualreviews.org

This articles doi:10.1146/annurev.biochem.77.061306.125255

Copyright c 2008 by Annual Reviews.All rights reserved

0066-4154/08/0707-0229$20.00

Key Words

DNA motor proteins, genome maintenance, meiosis,recombinases, recombination mediators

AbstractHomologous recombination (HR) serves to eliminate deleteriouslesions, such as double-stranded breaks and interstrand crosslinks,from chromosomes. HR is also critical for the preservation of repli-cation forks, for telomere maintenance, and chromosome segrega-tion in meiosis I. As such, HR is indispensable for the maintenanceof genome integrity and the avoidance of cancers in humans. TheHR reaction is mediated by a conserved class of enzymes termedrecombinases. Two recombinases, Rad51 and Dmc1, catalyze thepairing and shufing of homologous DNA sequences in eukaryoticcells via a lamentous intermediate on ssDNA called the presynapticlament. The assembly of the presynaptic lament is a rate-limitingprocess that is enhanced by recombination mediators, such as thebreast tumor suppressor BRCA2. HR accessory factors that facil-itate other stages of the Rad51- and Dmc1-catalyzed homologousDNA pairing and strand exchange reaction have also been identied.Recent progress on elucidating the mechanisms of action of Rad51and Dmc1 and their cohorts of ancillary factors is reviewed here.

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Contents

BIOLOGICAL FUNCTIONSOF HOMOLOGOUSRECOMBINATION. . . . . . . . . . . . . 230

HOMOLOGOUSRECOMBINATIONPATHWAYS ANDBIOLOGICAL RELEVANCE . . . 231

HR GENES AND PROTEINS . . . . . 233THE RAD51 RECOMBINASE

AND PRESYNAPTICFILAMENT FORMATION . . . . . 233

THE MEIOSIS-SPECIFICRECOMBINASE DMC1 . . . . . . . . 236

ROLE OF ATP BINDING ANDHYDROLYSIS INPRESYNAPTIC FILAMENTDYNAMICS. . . . . . . . . . . . . . . . . . . . . 236

OPPOSING EFFECTS OF RPA INPRESYNAPTIC FILAMENTASSEMBLY. . . . . . . . . . . . . . . . . . . . . . 236

CONSERVED FUNCTIONALATTRIBUTES OF THERECOMBINATIONMEDIATORS . . . . . . . . . . . . . . . . . . . 237

THE S. CEREVISIAE RAD52PROTEIN AND ITSRECOMBINATIONMEDIATOR ACTIVITY . . . . . . . . 237

OTHER FUNCTIONS OF THES. CEREVISIAE RAD52PROTEIN. . . . . . . . . . . . . . . . . . . . . . . 238

THE HUMAN RAD52 PROTEINAND ITS HR FUNCTION. . . . . . 239

THE HR ROLE OF THE TUMORSUPPRESSOR BRCA2 . . . . . . . . . . 239

SALIENT FEATURES OF BRCA2ORTHOLOGUES. . . . . . . . . . . . . . . 240

RECOMBINATION MEDIATOR

ACTIVITY OF U. MAYDISBRH2 AND HUMAN BRCA2PROTEINS . . . . . . . . . . . . . . . . . . . . . 242

A MODEL FOR THE BRCA2RECOMBINATIONMEDIATOR ACTIVITYAND SOME QUESTIONS . . . . . . 243

THE BRCA2-ASSOCIATEDPROTEINS DSS1 ANDPALB2 (FANCN) . . . . . . . . . . . . . . . . 243

THE S. CEREVISIAERAD55-RAD57 COMPLEXAND ITS RECOMBINATIONMEDIATOR ACTIVITY . . . . . . . . 244

THE HUMAN RAD51B-RAD51CCOMPLEX AND ITSRECOMBINATIONMEDIATOR ACTIVITY . . . . . . . . 245

THE S. POMBE SWI5-SFR1COMPLEX AND ITSRECOMBINATIONMEDIATOR ACTIVITY . . . . . . . . 245

RELATIONSHIP OF THES. CEREVISIAE MEI5-SAE3COMPLEX TO THE S. POMBESWI5-SFR1 COMPLEX . . . . . . . . . 246

BIPARTITE ACTION OF THEHOP2-MND1 COMPLEXIN RECOMBINASEENHANCEMENT . . . . . . . . . . . . . . 246

THE MULTIFUNCTIONALROLE OF THE DNA MOTORPROTEIN RAD54 IN HR . . . . . . . 247

RAD54-RELATED DNA MOTORPROTEINS: S. CEREVISIAERDH54 AND HUMANRAD54B . . . . . . . . . . . . . . . . . . . . . . . . . 249

CONCLUSIONS. . . . . . . . . . . . . . . . . . . 250

BIOLOGICAL FUNCTIONSOF HOMOLOGOUSRECOMBINATION

Homologous recombination (HR), theexchange of genetic information between

allelic sequences, has essential roles in meio-sis and mitosis. In meiosis, HR mediates theexchange of information between the mater-nal and paternal alleles within the gamete

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precursor cells and thus generates diversityamong the progeny derived from commonparents. HR has a second critical functionin meiosis; it ensures proper segregation ofhomologous chromosome pairs at the rstmeiotic division through the formation ofcrossovers, resulting in gametes with the cor-rect number of chromosomes. These func-tions of HR ensure stability of the organismkaryotype. Meiotic HR is initiated by Spo11-mediated DNA double-strand breaks (DSBs)(1). HR maintains somatic genomic stabil-ity by promoting accurate repair of DSBs in-duced by ionizing radiation and other agents,repair of incomplete telomeres that arise whenthe enzyme telomerase is nonfunctional, re-pair of DNA interstrand crosslinks, and therepair of damaged replication forks. Althoughcells have alternate DNA repair pathways suchas nonhomologous DNA end-joining, thesemay not be operative at all phases of the cellcycle, they do not always act on injured repli-cation forks, nor are they as precise as HR inthe repair of broken chromosomes.

Mutations in genes encoding the enzy-matic steps of HR result in extreme sensitiv-ity to DNA-damaging agents such as ionizingradiation in model organisms such as Saccha-romyces cerevisiae (2). Additionally, these mu-tant strains are defective in processes that in-volve the repair of naturally occurring DSBssuch as those breaks made during mating-typeswitching and during meiosis (3, 4). In ver-tebrate organisms, the equivalent mutationsare very often lethal, most likely reectingthe higher occurrence of spontaneous DSBsduring somatic growth and the essential rolethat HR plays in the repair of damaged DNAreplication forks (5, 6). HR in higher eukary-otes involves additional factors not found inall model organisms. Heritable mutations inthe cancer-prone disease Fanconi anemia andfamilial breast cancer have turned out to bein some of these factors (7). These are gener-ally hypomorphic mutations as the genes arefrequently essential.

The focus of this review is on the factorsthat promote HR through their action on the

Crossovers:recombinationproducts that entailan exchange of thearms of theparticipantchromosomes

Recombinase: anenzyme thatmediates the pairingand exchange ofDNA strands duringhomologousrecombination

Gene conversion:nonreciprocalhomologousrecombinationbetween an intactdonor duplex and agapped or brokenrecipient molecule

recombinases Rad51 and Dmc1. The Rad51recombinase mediates the formation of DNAjoints that link homologous DNA molecules.It is active in somatic and meiotic cells. Asecond recombinase, Dmc1, promotes similarassociations of homologous DNA molecules,but is active only in meiosis and acts in concertwith Rad51. Several DNA helicases have beenfound to either negatively regulate HR initia-tion or specically suppress crossover forma-tion. The biological roles of these DNA he-licases and their mechanism of action are thesubject of recent reviews (8, 9) and are notcovered here.

HOMOLOGOUSRECOMBINATION PATHWAYSAND BIOLOGICAL RELEVANCE

Many HR genes were rst identied bymutants that are hypersensitive to DNA-damaging agents that cause DSBs, and by afailure to give viable meiotic products (seebelow for a more detailed description). Fromstudies of these mutants using recombinationreporters, models of HR and classication ofHR pathways have emerged. These modelsare based on the repair of a DSB usinga homologous DNA sequence. The rstHR model for repair of a DSB was basedon observations of transformation in yeastusing linear plasmids that carried sequenceshomologous to yeast chromosomal DNA (10,11). This model, called the double-strandbreak-repair (DSBR) model, can explainmuch of the meiotic segregation in the fungiand linked crossing over and gene conversionas different outcomes of DSB repair (12).Although this model has been modiedfrom its original conception, subsequentmodels retain several key features. The mostimportant are (a) initiation of HR by a DSB,(b) processing of the DSB by nucleolytic re-section to give single-strand tails with 3-OHends, (c) formation of a recombinase lamenton the ssDNA ends, (d ) strand invasion intoa homologous sequence to form a D-loopintermediate, (e) DNA polymerase extension

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Holliday junction(HJ): a cruciformintermediategenerated late inhomologousrecombination.Resolution of theHolliday junctioncan result incrossover products

Replicationcheckpoints: signaltransductioncascades triggered bydamaged replicationforks and that lead tocell cycle arrest ordelay

from the 3 end of the invading strand, ( f )capture of the second DSB end by annealingto the extended D loop, ( g) formation of twocrossed strand or Holliday junctions (HJ)s,and lastly (h) resolution of the HJs to givecrossover or noncrossover products.

Although the DSBR model explains manyobservations related to meiotic recombina-tion products in the fungi, one of its maintenets, the linking of gene conversion andcrossovers through the resolution of a com-mon intermediate, is not supported by mitoticrecombination data where most DSB repair ismost frequently unassociated with crossovers.To keep the original DSBR model for mi-totic HR would necessitate the imposition ofstrict rules on HJ resolution in a noncrossovermode only. A second model that avoids thisrestriction and is based on mitotic DSB re-pair data in model organisms has been de-veloped (1315). The essence of this modelis a migrating D loop that never leads tocapture of the second DSB end. Instead, af-ter the initial steps of DSB resection, DNAstrand invasion, and repair DNA synthesis,the invading strand is displaced and annealswith the second resected DSB end. Becauseno HJ is formed, only noncrossover prod-ucts are made. Since the model involves DNAsynthesis followed by strand annealing, it iscalled synthesis-dependent strand annealing(SDSA). Although the SDSA model was ini-tially developed to explain mitotic DSB repair,there is now substantial evidence to suggestthat SDSA is also important during meioticHR. Not all meiotic DSBs result in crossoverproducts: only a small fraction of these breaksdo. The existing data suggest that there aretwo waves of meiotic DSB-promoted HR.The rst wave proceeds by SDSA and is onlynoncrossover, whereas the second wave pro-ceeds by DSBR, forms double HJs, and ismainly, if not solely, crossover.

Sometimes a DSB is closely anked bydirect repeats. This DNA organization pro-vides the opportunity to repair the DSB by adeletion process using the repeated DNA se-quences, called single-strand annealing (SSA)

(1618). In the SSA process, the DSB ends areresected, but then instead of engaging a ho-mologous DNA sequence for strand invasion,the resected ends anneal to each other. Theprocess is nished by nucleolytic removal ofthe protruding single-strand tails, and resultsin deletion of the sequences between the di-rect repeats and also one of the repeats. Sincestrand invasion is not involved, SSA is inde-pendent of strand invasion and HJ resolutionfactors (19).

Some DSBs, such as those that can occurat telomeres or at broken replication forks,are single-ended (2022). These too can par-ticipate in HR, through a single-ended in-vasion process called break-induced replica-tion (BIR) (2329). In BIR, the DSB end isnucleolytically processed similar to the re-section that occurs in other DSB HR repairevents. The single-strand tail then invades ahomologous DNA sequence, often the sis-ter chromatid or homolog chromosome butsometimes a repeated sequence on a differ-ent chromosome. The invading end is usedto copy information from the invaded donorchromosome by DNA synthesis. When thesister chromatid or homolog chromosome isused, the repair is accurate. When a repeatedsequence on a nonhomologous chromosomeis engaged to initiate repair, the result is a non-reciprocal translocation. Most BIR events aredependent on the HR factors used in DSBRand SDSA, but a small fraction can occur inde-pendently of these factors that include Rad51.BIR is often used to repair broken or short-ened telomeres (26, 29).

The requirement for HR in DNA repli-cation is highlighted by the nding thatmany replication mutants and mutants infactors required for checkpoint activationwhen replication is stalled are dependent onHR genes for viability (3032). This ndingsuggests that replication checkpoints preventHR at stalled or damaged forks by stabilizingthe replication complex at the fork, thusavoiding the occurrence of HR-promotingor HR-like intermediates. The nding alsosuggests that defective replication can result

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in HR-provoking intermediates, e.g., gaps ator behind the replication fork. Because HR atstalled replication forks can lead to genomicrearrangements, it might be expected that itwould be tightly controlled. In the case ofcollapsed replication forks, HR is used forone-ended strand invasion events using thesister chromatid to reconstruct the fork. Thisprocess may be promoted by sister chromatidcohesion complexes.

HR GENES AND PROTEINS

A large proportion of the genes needed forHR were initially identied in the buddingyeast S. cerevisiae by the classical procedure ofmutant isolation (typically based on sensitiv-ity of mutant cells to DNA-damaging agentssuch as ionizing radiation), in-depth epistasisanalyses of the available mutants, and cloningof the corresponding genes by complementa-tion of the mutant phenotype. These genesare collectively known as the RAD52 epistasisgroup. The structure and function of the pro-teins encoded by genes of the RAD52 groupare highly conserved among eukaryotes, fromyeast to humans. Table 1 lists the membersof the RAD52 gene group as rst dened inS. cerevisiae and their human equivalent.

Generally speaking, in addition to me-diating mitotic HR events, members of theRAD52 group are needed for meiotic recom-bination as well. Aside from the RAD52 coregroup, a whole host of genes that uniquely af-fect meiotic recombination have been uncov-ered in screens designed to search for them(1). For instance, the DMC1 gene, which en-codes one of the two recombinase enzymes,was identied as a cDNA species that isstrongly upregulated when S. cerevisiae cellsenter into meiosis. Overall, more is knownabout the properties of the mitotic HR factorsthan of meiosis-specic factors. We discussrecent progress in understanding the mecha-nism of the HR machinery without providingan exhaustive account on the properties of allthe known HR factors (reviewed in 4, 8, 19,33).

Epistasis group: agroup of genes thatfunction in the samebiological pathway.Epistasis isestablished by doublemutant analyses

Presynapticfilament: theright-handed helicalrecombinaselament that isassembled on ssDNA

THE RAD51 RECOMBINASEAND PRESYNAPTICFILAMENT FORMATION

The enzymes that mediate the pairing andshufing of DNA sequences during HR arecalled recombinases, and the reaction medi-ated by these enzymes is termed homologousDNA pairing and strand exchange. Two re-combinases, Rad51 and Dmc1, exist in eu-karyotes. Rad51 is needed for mitotic HRevents such as DSB repair and also for mei-otic HR, whereas Dmc1 is only expressed inmeiosis so its function is restricted therein.The salient attributes of the DMC1 gene andencoded protein are discussed in a separatesection.

Much of our knowledge on the RAD51gene and its encoded protein has been de-rived from genetic and biochemical stud-ies done in S. cerevisiae. The S. cerevisiaerad51 mutants are highly sensitive to DNA-damaging agents and show defects in mitoticand meiotic recombination. Analysis of theS. cerevisiae RAD51 gene, which was clonedindependently by three different groups, re-vealed signicant homology of its encodedprotein to the bacterial recombinase RecA,with particular conservation of those RecAresidues that are critical for its recombi-nase function, including DNA binding andATP hydrolysis (3436). The structure of theRad51 protein has been conserved amongeukaryotes. Whereas S. cerevisiae rad51 mu-tants are viable mitotically, ablation of theRAD51 gene in vertebrates engenders mitoticlethality (19), which likely reects an essen-tial role of Rad51-mediated HR in the repairof damaged DNA replication forks and hencethe successful navigation through S phase.

Rad51 and its prokaryotic counterpartRecA exists as a homo-oligomer in solution,being heptameric and hexameric, respectively(33, 37). Just as in the case of RecA, with ATP(or an analogue of ATP) available, S. cerevisiaeRad51 protein assembles onto ssDNA or ds-DNA to form a right-handed helical poly-mer that can span thousands of bases or base

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Table 1 Homologous recombination factors

Human S. cerevisiae Biochemical function Additional featuresProteins that function with Rad51

MRN complex:Mre11-Rad50-Nbs1

MRX complex:Mre11-Rad50-Xrs2

DNA bindingNuclease activities

Involved in DNA-damage checkpointsAssociated with DSB end resection

BRCA2 (none) ssDNA bindingRecombination mediator

Interacts with RPA, Rad51, Dmc1,PALB2, DSS1

Member of the Fanconi anemia groupRad52a Rad52 ssDNA binding and annealing

Recombination mediatorInteracts with Rad51 and RPA

?b Rad59 ssDNA binding and annealing Interacts with Rad52Homology to Rad52

Rad54Rad54B

Rad54Rdh54

ATP-dependent dsDNA translocaseInduces superhelical stress in dsDNAStimulates the D-loop reaction

Member of the Swi2/Snf2 proteinfamily

Chromatin remodelerInteracts with Rad51The yeast proteins remove Rad51 from

dsDNARad51B-Rad51CRad51D-XRCC2Rad51C-XRCC3

Rad55-Rad57 ssDNA bindingRecombination mediator activity

(Rad55-Rad57 & Rad51B-Rad51C)

Rad51B-Rad51C and Rad51D-XRCC2 form a tetrameric complex

Rad51C associates with aHolliday-junction resolvase activity

Hop2-Mnd1 Hop2-Mnd1 Stimulates the D-loop reactionStabilizes the presynaptic lamentPromotes duplex capture

Interacts with Rad51 and Dmc1

Proteins that function with Dmc1Hop2-Mnd1 Hop2-Mnd1 Stimulates the D-loop reaction

Stabilizes the presynaptic lamentPromotes duplex capture

Interacts with Dmc1 and Rad51

?b Mei5-Sae3 Predicted recombination mediatoractivity

Interacts with Dmc1Likely functional equivalent ofS. pombe Sfr1-Swi5

Rad54B Rdh54 Stimulates the D-loop reaction Interacts with Dmc1 and Rad51

aRecombination mediator activity has been found in the yeast protein only.bNo human equivalent has been identied.

pairs. The Rad51-DNA nucleoprotein la-ment harbors 1819 bases or base pairs ofDNA and 6 protein monomers per helicalturn. It has a pitch of close to 100 A, corre-sponding to an axial rise of 5.2 to 5.5 A perbase or base pair (38, 39). The DNA in thislamentous structure is therefore held in ahighly extended conformation, i.e., stretchedabout 50% when compared to a naked B formduplex molecule. Human Rad51 forms heli-cal laments on both ssDNA and dsDNA that

exhibit biophysical attributes similar to thosedescribed for S. cerevisiae Rad51 (40). Bio-chemical studies have provided clear evidencethat only the Rad51-ssDNA nucleoprotein l-ament species is able to catalyze DNA jointformation (39), which supports the notionthat HR in cells is initiated via recruitment ofRad51 to the ssDNA generated via nucleolyticprocessing of DSBs (Figure 1) or ssDNAthat is associated with stalled or damagedDNA replication forks. The Rad51-ssDNA

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nucleoprotein lament is often referred to asthe presynaptic lament, and the biochem-ical steps that lead to the assembly of theRad51 lament are collectively known as thepresynaptic stage. The formation of Rad51-dsDNA laments with bulk chromatin coulddiminish the pool of Rad51 available for HRreactions. As discussed below, the S. cere-visiae Rad54 and Rdh54 proteins dissociateRad51-dsDNA laments, an activity that islikely critical for the intracellular recycling ofRad51.

Once assembled, the presynaptic lamentcaptures a duplex DNA molecule and searchesfor homology in the latter. From studies donewith RecA (33), it is expected that the homol-ogy search process occurs by way of randomcollisions between the presynaptic lamentand the duplex molecule. Thus, segments ofthe duplex are bound and tested in a reiterativefashion until homology is found. Upon the lo-cation of homology in the duplex molecule,the presynaptic lament is able to form DNAjoints that are either paranemic or plec-tonemic in nature. In the paranemic joint, aninternal region of the ssDNA is paired withthe duplex molecule via canonical Watson-Crick hydrogen bonds, but the paired DNAstrands are not topologically linked. Studiesdone in the Radding laboratory have shownthat the paranemic linkage mostly involves theformation of AT base pairs between the re-combining DNA molecules (41). The three-stranded, paranemically paired nucleoproteinintermediate is referred to as the synapticcomplex. Recently published work has pro-vided evidence for a role of the Hop2-Mnd1protein complex in functionally synergizingwith the presynaptic lament in the capture ofduplex DNA and the assembly of the synapticcomplex (42, 43) (see below). Although rela-tively short-lived, the paranemic joint facili-tates the location of a free DNA end to ini-tiate the formation of a plectonemic joint, inwhich the participant DNA strands are boundby Watson-Crick hydrogen bonds and topo-logically intertwined (44, 45). The nascent

Double-strand break

End resection

Strand invasionDNA synthesis

D-loop dissociationAnnealing

DNA synthesisLigation

Noncrossover

SDSA Second end captureDNA synthesisLigation

Noncrossoveror

Crossover

HJ resolution

DSBR

c

3'

DNA damagea

b

3'

Figure 1Pathways of DNA double-strand break repair by homologousrecombination. Double-strand breaks (DSBs) can be repaired by distinctivehomologous recombination (HR) pathways, such as synthesis-dependentstrand annealing (SDSA) and double-strand break repair (DSBR). (a) AfterDSB formation, the DNA ends are resected to yield 3 single-strand DNA(ssDNA) overhangs, which become the substrate for the HR proteinmachinery to execute strand invasion of a partner chromosome. After asuccessful homology search, strand invasion occurs to form a nascentD-loop structure. DNA synthesis then ensues. (b) In the SDSA pathway, theD loop is unwound and the freed ssDNA strand anneals with thecomplementary ssDNA strand that is associated with the other DSB end.The reaction is completed by gap-lling DNA synthesis and ligation. Onlynoncrossover products are formed. (c) Alternatively, the second DSB endcan be captured to form an intermediate that harbors two Hollidayjunctions (HJ)s, accompanied by gap-lling DNA synthesis and ligation.The resolution of HJs by a specialized endonuclease can result in eithernoncrossover (black triangles) or crossover products (gray triangles).

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Synaptic complex:the ternary complexof the recombinaselament, ssDNA,and dsDNA in whichthe DNA moleculesare held inhomologous registry

Recombinationmediator: a proteinthat facilitates theassembly of therecombinasepresynaptic lamentvia RPAdisplacement fromssDNA

plectonemic joint can be extended by DNAstrand exchange being catalyzed by the presy-naptic lament. The DNA strand exchangereaction is facilitated by the Rad54 protein(46). Moreover, Rad54 also promotes a spe-cialized form of DNA strand exchange thatinvolves the formation of a HJ and migrationof the branch point in the HJ (47).

Nucleation of Rad51 onto ssDNA is a slowprocess, which renders presynaptic lamentassembly prone to interference by the ssDNAbinding protein RPA. Certain recombinaseaccessory factors, which have been termedrecombination mediators and include the tu-mor suppressor BRCA2, can overcome the in-hibitory effect of RPA on the assembly of theRad51 presynaptic lament. As such, these re-combination mediators are critical for the ef-ciency of HR in vivo. We expand on the mech-anism of action of the known recombinationmediators below.

THE MEIOSIS-SPECIFICRECOMBINASE DMC1

The DMC1 gene was isolated by Bishop et al.(48) in a screen for cDNA species specicfor S. cerevisiae meiosis. The DMC1-encodedprotein is present in almost all eukaryotes in-cluding humans and is structurally related toRecA and Rad51 (48, 49). Ablation of DMC1in S. cerevisiae, Arabidopsis thaliana, and miceproduces a constellation of meiotic abnor-malities that reect an indispensable role ofthe Dmc1 protein in meiotic recombinationand chromosome segregation (1, 48, 50, 51).Dmc1 exists as an octamer in solution (52),and recent biochemical studies have providedcompelling evidence that it too forms right-handed, helical laments on ssDNA in anATP-dependent manner and catalyzes the ho-mologous DNA pairing and strand exchangereaction within the context of these nucle-oprotein laments (5355). Thus, in its ac-tion as a recombinase, Dmc1 possesses thesame functional attributes as have been docu-mented for RecA and Rad51.

ROLE OF ATP BINDING ANDHYDROLYSIS IN PRESYNAPTICFILAMENT DYNAMICS

Even though Rad51 and Dmc1 hydrolyzeATP, especially when DNA bound (5659),ATP hydrolysis is not needed for the assem-bly of the presynaptic lament. In fact, bio-chemical studies have provided evidence thatATP hydrolysis within the microenvironmentof the presynaptic lament leads to the disso-ciation of recombinase molecules from DNA(53, 6062). As a consequence, the use of anonhydrolyzable nucleotide analogue (such asAMP-PNP) (61, 62), calcium ion (53, 60, 62),or a Rad51 variant that binds ATP but lacksATPase activity (61) leads to the stabilizationof the presynaptic lament. That ATP hydrol-ysis promotes the turnover of the presynapticlament is also a well-known property of RecA(33, 63). The ATP hydrolysis-linked turnoverof the presynaptic lament could promotethe intracellular recycling of the recombinases(i.e., preventing the nonproductive associa-tion of the recombinase protein with DNA)and to make available the primer end in thenewly made D loop to initiate the repair DNAsynthesis reaction.

OPPOSING EFFECTS OFRPA IN PRESYNAPTICFILAMENT ASSEMBLY

The heterotrimeric RPA (replication proteinA) is an abundant nuclear protein that bindsssDNA with high afnity and can remove sec-ondary structure in ssDNA. Depending onthe circumstances, RPA can exert a stimu-latory or an inhibitory effect on the assem-bly of the presynaptic lament (64, 65). Thestimulatory action of RPA was noted in 1994when the recombinase activity of S. cerevisiaeRad51 was rst reported (59). Subsequentstudies have provided evidence that RPA facil-itates the assembly of the presynaptic lamentvia the removal of secondary structure in thessDNA (66) and also by sequestering ssDNAgenerated during the homologous DNA

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pairing and strand exchange reaction (67, 68).However, an amount of RPA that is sufcientto saturate the available ssDNA (the ssDNAbinding site size of RPA is 25 nucleotides perheterotrimeric molecule) strongly suppressesthe ssDNA-dependent ATPase and recombi-nase activities of Rad51 and Dmc1 (64, 65, 6971). That RPA can exclude the recombinasesfrom the HR substrate has been validated instudies that employed chromatin immuno-precipitation (ChIP) and cytological methods.Several recombination mediators have beenshown to counteract the inhibitory action ofRPA (see below).

CONSERVED FUNCTIONALATTRIBUTES OF THERECOMBINATION MEDIATORS

As mentioned above, the assembly of thepresynaptic lament can be severely impededby RPA. Studies in several laboratories haveled to the identication of recombination me-diators capable of overcoming this inhibitoryaction of RPA. Specically, the addition ofthese recombination mediators with the re-combinase protein to an RPA-coated ssDNAtemplate permits the efcient assembly of thepresynaptic lament (19, 64, 6972). The re-combination mediator activity of a variety ofHR factors has also been demonstrated usingthe restoration of the recombinases ssDNA-dependent ATPase as the readout (70, 71) orby electron microscopy to directly visualizetheir effect on presynaptic lament formation(72). Mutations in the recombination medi-ators invariably impair the delivery of theircognate recombinase to the HR substrate incells, as revealed in cytological and ChIP ex-periments (19, 7377). Since the K45E muta-tion in the largest RPA subunit is associatedwith a synapsis defect, RPA likely also plays arole in DNA strand invasion during HR (78).

The recombination mediators share com-mon features in that they are all capableof physically interacting with their cognaterecombinase and bind ssDNA preferentiallyover dsDNA (19, 71, 72, 79). In some in-

Chromatin im-munoprecipitation(ChIP): a powerfultechnique fordetermining whethera protein associateswith a specic regionof the genome

stances, an interaction of the recombinationmediators with RPA has also been noted (8083). Only a catalytic quantity of the recom-bination mediators is needed to see reversalof RPA inhibition, which is very likely dueto the fact that the addition of recombinasemolecules to a nascent presynaptic lament(i.e., lament growth) is sufcient to displaceRPA from ssDNA (84). The genetic char-acteristics and salient features of the variousknown recombination mediators are reviewedbelow.

THE S. CEREVISIAE RAD52PROTEIN AND ITSRECOMBINATIONMEDIATOR ACTIVITY

The S. cerevisiae Rad52 protein has been themost intensely studied recombination media-tor to date. Genetically speaking, S. cerevisiaerad52 mutants are extremely sensitive to a va-riety of DNA-damaging agents and engendera general defect in all the known pathwaysof HR, including Rad51-independent reac-tions such as SSA (see below). Rad52 is a ring-shaped oligomer (81, 85), and oligomerizationof monomers to form the protein ring is medi-ated by the N-terminal portion of the protein(86). Higher-order multimeric structures ofRad52 have also been documented (86). Theinclusion of a catalytic quantity of Rad52 leadsto highly efcient reversal of RPA-imposedinhibition of the ssDNA-dependent ATPaseand recombinase activities of Rad51 (65, 69,70). In both mitotic and meiotic cells, the re-cruitment of Rad51 to DSBs is strongly de-pendent on Rad52, but the DSB recruitmentof Rad52 shows no dependence on Rad51 (73,7577). Taken together, the genetic and bio-chemical studies on S. cerevisiae Rad52 pro-vide compelling evidence that it helps deliverRad51 to the ssDNA substrate during HR.

That Rad52 physically associates withthe Rad51 protein was rst noted in ayeast two-hybrid protein-protein interactionanalysis conducted by Milne et al. (87),and this was subsequently conrmed by

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coimmunoprecipitation of the two proteinsfrom cell extract (65) and also using pu-ried proteins in afnity pulldown assays(36). The Rad51 interaction domain resideswithin the carboxyl-terminal portion of theRad52 protein (87), and truncation mutations(rad52 327 and rad52 409412) that abol-ish Rad51 binding attenuate the recombina-tion mediator activity of the latter (70, 88) andcompromise HR deciency in both mitoticand meiotic cells (88, 89). These observationssupport the premise that complex formationwith Rad51 is indispensable for the recombi-nation mediator activity of Rad52. The HRdeciency of the rad52 409412 mutant canbe largely overcome by the overexpression ofRad51 (88), indicating that the recombina-tion mediator function of ScRad52 can be by-passed when the intracellular concentration ofRad51 is elevated.

Mortensen et al. (90) rst reported thatScRad52 has a DNA binding activity thatis specic for ssDNA. These authors foundthat the N-terminal one third of Rad52 har-bors a DNA binding function, and exten-sive subsequent studies have focused on howthe N-terminal portion of human Rad52 en-gages ssDNA (9194). The structure of theconserved N-terminal protein oligomeriza-tion/DNA binding domain of the humanRad52 protein has been analyzed by X-raycrystallography (91, 94). The crystallographicdata reveal an undecameric (11-subunit) ringstructure with a deep groove on the outersurface, with an abundance of basic and aro-matic residues lining this groove (91, 94). De-tailed mutational analyses have provided ev-idence for the involvement of these residuesin DNA engagement (91, 92). Mutations ofthe equivalent residues in the ScRad52 pro-tein compromise DNA repair and HR ef-ciency, attesting to the importance of theN-terminal DNA binding domain in Rad52protein function (95). It remains to be deter-mined how these N-terminal mutations af-fect the recombination mediator activity ofScRad52. Recent studies in the laboratory ofthe authors have led to the identication of a

second DNA binding domain in the carboxylterminal portion of ScRad52 (L. Krejci, C.Seong & P. Sung, unpublished observation).How this second DNA binding domain con-tributes to the known functions of ScRad52 isthe subject of ongoing investigations.

Rad52 protein also associates with RPAin S. cerevisiae cells (80), and it appears thatboth the largest and middle subunits of RPAare able to directly bind Rad52 (81, 96).The ability to interact with RPA is con-served in the human Rad52 protein (96). Be-cause Rad52 is unable to overcome inhibitionposed by the Escherichia coli SSB protein onRad51-mediated homologous DNA pairingand strand exchange (69), specic associationof Rad52 with RPA is very likely necessary forthe recombination mediator activity of Rad52.

A model for the recombination mediatorfunction of ScRad52 is presented in Figure 2.Among the most pertinent questions regard-ing the recombination mediator function ofRad52 are the relative contributions of the N-terminal and C-terminal DNA binding do-mains of this protein and the relevance ofprotein oligomerization. In this regard, itshould be noted that the N-terminal domainof ScRad52 is important for DNA annealingreactions during yeast meiosis (97).

OTHER FUNCTIONS OF THES. CEREVISIAE RAD52 PROTEIN

Even though the rad52 327 and 409412alleles encode proteins that are defective inRad51 interaction and consequently lack re-combination mediator activity (70, 88), theyare not as decient in mitotic and meiotic HRas is the rad52 null mutant (88, 89). Moreover,the rad52 327 protein retains residual abilityto mediate the recruitment of Rad51 to DSBs(C. Seong & P. Sung, unpublished results).Clearly then, in HR events that are Rad51 de-pendent, Rad52 protein serves an undenedrole that is distinct from its well-characterizedrecombination mediator activity. Aside fromparticipating in Rad51-dependent HR events,Rad52 is also required for Rad51-independent

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SSA and BIR reactions (19, 26). Consistentwith its involvement in SSA, Rad52 pro-tein anneals DNA strands that are nakedor coated with RPA (81, 82). It has beeninferred that Rad52-mediated annealing ofRPA-coated DNA strands is relevant for thecapture of the second DNA end in the DSBRpathway of HR (98). Rad52 is absent in iesand worms but present in other eukaryotes,including humans.

THE HUMAN RAD52 PROTEINAND ITS HR FUNCTION

Even though the human Rad52 protein re-sembles its S. cerevisiae counterpart in struc-ture and biochemical attributes (i.e., it isoligomeric and able to bind ssDNA and pro-mote ssDNA annealing), and can under somespecialized circumstances enhance Rad51-mediated homologous DNA pairing (99), arecombination mediator activity has not yetbeen demonstrated for it. In fact, extensive ef-forts in the laboratory of the authors (P. Sung,unpublished observations) have failed to re-veal such an activity. The apparent lack of arecombination mediator activity in hRad52may explain why it plays only a subsidiaryrole in HR in vertebrates (19, 100). Alterna-tively, hRad52 may augment the recombina-tion mediator activity of other factors such asBRCA2 or complexes of the Rad51 paralogues(see below). In this regard, synthetic lethalityhas been noted for a double mutant of Rad52and XRCC3 (a Rad51 paralogue; see below)(100). It of course also remains possible that arecombination mediator activity will only berevealed upon the posttranslational modica-tion (such as phosphorylation) of hRad52 orthe inclusion of a partner protein.

THE HR ROLE OF THE TUMORSUPPRESSOR BRCA2

Mutations in the BRCA2 gene predisposethe affected individuals to breast, ovarian,and other cancers (101). Biallelic inactivationof BRCA2 can cause the cancer-prone dis-

Rad51

Rad51 & RPA

Rad51 alone

Rad52

RPA Rad51DNA binding

DNA bindingOligomerization

Functionaldomain:

a

b

c

Rad52 RPA

Key:

Figure 2Rad52 and its role as a recombination mediator. (a) In the presence of ATP,Rad51 forms the presynaptic lament on ssDNA, but is unable to do so inthe presence of replication protein A (RPA). (b) The functional domains inS. cerevisiae Rad52 are indicated. (c) In its recombination mediator role,Rad52 forms a complex with Rad51 and delivers it to RPA-coated ssDNAto seed the assembly of the presynaptic lament. The polymerization ofadditional Rad51 molecules results in the further displacement of RPA fromthe DNA.

Paralogue: aprotein that is relatedto another protein inprimary structurebut not necessarily infunction and thatarises through geneduplication

ease Fanconi anemia (102) (see sidebar: Con-nection between Fanconi Anemia and Ho-mologous Recombination). Cells from ver-tebrate, plant, nematode, and fungal speciesdecient in BRCA2 or its orthologue aresensitive to DNA-damaging agents and im-paired for homology-directed elimination ofchromosome damage, including interstrandcrosslinks and DSBs (51, 101, 103109). Thetransport of RAD51 to the nucleus is im-paired in cells that harbor a cancer-associated

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Orthologue: thestructural andfunctional equivalentof a protein in adifferent species

CONNECTION BETWEEN FANCONIANEMIA AND HOMOLOGOUSRECOMBINATION

Fanconi anemia (FA) is an autosomal recessive chromosomefragility syndrome characterized by progressive bone marrowfailure, short stature, cancer predisposition, and a severe de-ciency in the removal of lesions, interstrand crosslinks inparticular, from DNA (209, 210). FA is a complex multigenicdisorder comprised of 13 complementation groups (FA- A, B,C, D1, D2, E, F, G, I, J, L, M, and N), and each of the corre-sponding genes has been identied. FA-D1 and FA-N patientsharbor biallelic mutations in BRCA2 and its associated pro-tein PALB2, respectively, thus revealing a functional linkage ofBRCA2-/PALB2-dependent homologous recombination tothe FA pathway of DNA-damage repair and response (102,209, 210). Upon DNA damage, a complex of several FANCproteins helps mediate the monoubiquitination of FANCD2and FANCI, resulting in the chromatin localization of the lat-ter two proteins. It is believed that the chromatin-bound ubiq-uitinated FANCD2-FANCI complex recruits BRCA2/PALB2and associated proteins, such as Rad51, to initiate DNA repairby a homology-directed mechanism (102, 209212). BecauseFA cells are frequently impaired for HR (209, 210), some ofthe FA proteins may possibly inuence the homologous re-combination process via BRCA2/PALB2.

BRCA2 truncation (110). Furthermore, ham-ster Brca2 mutant cells exhibit an abnormalS phase DNA-damage checkpoint response(104). Brca2-decient mice suffer embry-onic lethality, an observation that highlightsthe importance of BRCA2-dependent DNA-damage repair and response (107). There isalso good evidence that BRCA2 is impor-tant for meiotic HR (51, 103, 105, 111). Alarge body of results point to an importantrole of BRCA2 in the assembly of presynap-tic laments of Rad51. Specically, and asexpounded below, BRCA2 and its ortholo-gous proteins bind DNA (71, 72, 79, 105),physically interact with Rad51 (101, 107, 112,113), and are needed for the formation ofDNA damageinduced nuclear Rad51 foci(101, 114). Importantly, studies on the Usti-lago maydis Brh2 protein (the BRCA2 ortho-

logue in that organism) and a polypeptidethat harbors critical functional domains ofthe human BRCA2 protein have provided di-rect biochemical evidence for a recombina-tion mediator activity (71, 72) (see below). Anability of human BRCA2 and its Arabidopsisthaliana orthologue to interact with Dmc1 hasalso been demonstrated (115, 116), althoughwhether BRCA2 serves a recombination me-diator role in Dmc1-mediated HR reactionsremains to be explored. Notably, S. cerevisiaeand Schizosaccharomyces pombe do not possess aBRCA2-like molecule.

SALIENT FEATURES OF BRCA2ORTHOLOGUES

The BRCA2 orthologues vary greatly in size,ranging from 3418 amino acid residues ofthe human protein to only 383 residues ofthe Caenorhabditis elegans counterpart (calledBRC-2). Human BRCA2 utilizes two sepa-rate means to interact with Rad51 protein,through a reiterated motif called the BRC re-peat (101, 112, 113, 117) and also via a struc-turally distinct motif located at its extremecarboxyl terminus (107, 118); the carboxylterminal Rad51 binding domain will be re-ferred to as the CTRB domain henceforth.The BRC repeat, which comprises about30 amino acid residues, is highly conservedamong the BRCA2 orthologues (117, 119), al-though there is extreme variance in the num-ber of copies of this motif among the ortho-logues. For example, human BRCA2 harborseight BRC repeats, whereas Ustilago maydisBrh2 protein and A. thaliana Brca2 proteinpossess a single BRC repeat and four such re-peats, respectively. While most of the BRCrepeats from different BRCA2 orthologuesbind Rad51 with varying afnity, the BRC5and BRC6 repeats of human BRCA2 do notseem to be functional in this regard (112, 113).The second BRC repeat of the A. thalianaBrca2 protein appears to interact with Dmc1but not Rad51 (115). Biochemical and crys-tallographic approaches and also electron mi-croscopy have been employed to dene the

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nature of the BRC-Rad51 complex. The re-sults from these studies indicate that the BRCrepeat interacts with the monomeric form ofRad51 [Rad51 by itself is heptameric in solu-tion (37)] in the absence of DNA (120). Theatomic structure of a complex between BRC4of human BRCA2 and the RecA-homologydomain of Rad51 has been solved by X-raycrystallography (121). The structure revealsthat BRC4 mimics the motif in Rad51 thatmediates protein homo-oligomerization, thusproviding a structural basis as to why BRCbinds the monomeric form of Rad51 (121).Yang et al. (71) have suggested that bindingof the BRC repeat to Rad51 leaves one ofthe Rad51 oligomerization interfaces avail-able for the recruitment of another Rad51molecule, which could provide a means for thenucleation of Rad51 presynaptic lament as-sembly in the recombination mediator role ofBRCA2. Electron microscopy with accompa-nying three-dimensional reconstruction anal-yses have provided evidence that the BRCrepeat can interact with a preformed Rad51presynaptic lament without disruption of thelament, and that BRC3 and BRC4 of hu-man BRCA2 interact with different surfacesof Rad51 in the context of the presynaptic l-ament (122). The sole BRC repeat of the C.elegans BRC-2 protein is bipartite in struc-ture; one region is specic for monomericRad51 and the other interacts specically withand stabilizes the Rad51 presynaptic lament(123).

As mentioned above, BRCA2 also bindsRad51 through the CTRB domain (107, 118).An allele of mouse Brca2 lacking the CTRBdomain is associated with reduced viabil-ity, hypersensitivity to the DNA interstrandcrosslinking agent mitomycin C, chromoso-mal instability (124), and a modest impair-ment of the HR-directed repair of a site-specic DSB (106). Thus, the CTRB domainappears to make a signicant contribution to-ward the HR function of BRCA2 but is notessential for this function. Unlike the BRCrepeat, the CTRB domain interacts with theoligomeric form of Rad51 in the absence of

DNA (125, 126), and appears to bind theinterface of two adjacent Rad51 monomersin the context of the presynaptic lament(126). The CTRB domain harbors a serineresidue (serine 3291) that becomes phospho-rylated by cyclin-dependent kinases (CDKs)in a cell cycle-dependent fashion, a modica-tion that blocks the interaction with Rad51. Ithas been suggested that the CDK-mediatedphosphorylation of the CTRB domain servesas a molecular switch in regulating Rad51-mediated recombination (127). Note that theU. maydis Brh2 protein also harbors a Rad51binding domain in its carboxyl terminus. ThisC-terminal Rad51 binding domain, which iscrucial for the HR and DNA repair func-tions of Brh2 in vivo, is unrelated to the BRCrepeat (128). Whether this Brh2 domain isstructurally related to the CTRB domain ofBRCA2 is unclear. Dmc1 also interacts withthe CTRB domain in human BRCA2, albeitmore weakly than Rad51 (116).

Although human BRCA2 and its A.thaliana equivalent both interact with Dmc1,the primary Dmc1 binding domain appears tobe different in the two proteins. None of theBRC repeats in the human BRCA2 proteinhas any afnity for Dmc1, and the main Dmc1interaction site in human BRCA2 has beenmapped to a 26-amino-acid region (residues23862411) termed the PhePP motif. Thismotif is highly conserved in mammalianBRCA2 species but absent in the BRCA2 or-thologue in A. thaliana and other eukaryotes(116). In contrast, A. thaliana Brca2 associateswith Dmc1 through BRC2, the second of fourBRC repeats in this protein (115).

Aside from Rad51 interaction, BRCA2 hasalso been found to associate with RPA (83) byseveral biochemical criteria. RPA binding ismediated by the extreme N-terminal regionof BRCA2, and a cancer-associated mutation,Y42C, in BRCA2 attenuates or abolishes theability to bind RPA (83).

The DNA binding activity of BRCA2was rst revealed in a combined biochemi-cal and structural examination of the mouseBrca2 (mBrca2) protein by Yang et al. (79).

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The BRCA2 DNA binding domain (BRCA2DBD) is thought to harbor ve subdomains:three oligonucleotide/oligosaccharide-bind-ing (OB) folds (OB1, OB2, and OB3), a helix-turn-helix motif that is appended to OB2and called the Tower domain, and a proxi-mal alpha-helical region termed the helicaldomain (79). The OB folds of BRCA2 arestructurally similar to the OB folds presentin RPA. Embedded within the helical domainand OB1 is a surface that mediates complexformation with a small, highly acidic pro-tein DSS1 (see below for a more detailed de-scription of DSS1). The mBrca2 DBD-Dss1complex binds ssDNA but not dsDNA andappears to recognize the duplex-ssDNA tran-sition in various DNA substrates (79). In thisregard, the U. maydis Brh2-Dss1 complex hasa high afnity for a duplex-ssDNA junctionthat bears the 3 overhang polarity of a re-sected DSB (71). Crystallographic and bio-chemical evidence has implicated all threeOB folds of BRCA2 in DNA engagement(79).

RECOMBINATION MEDIATORACTIVITY OF U. MAYDIS BRH2AND HUMAN BRCA2 PROTEINS

The U. maydis Brh2 protein in complex withDss1 was found to possess a recombinationmediator activity (71). Using several biochem-ical approaches, Yang et al. demonstratedthat an amount of Brh2-Dss1 substoichio-metric to that of the U. maydis Rad51 pro-tein is sufcient to seed the assembly ofthe presynaptic lament on DNA precoatedwith RPA. The Brh2-Dss1 pair is particu-larly adept at mediating Rad51 lament as-sembly at a duplex-ssDNA junction that has a3 overhang polarity, which is in congruencewith the ability of this protein complex tospecically recognize such a DNA junction.These results suggest that Brh2-Dss1 pref-erentially seeds Rad51 presynaptic lamentassembly at the duplex-ssDNA junction of aresected DSB or another DNA lesion in vivo.

The sole BRC repeat of Brh2 can interactwith the human Rad51 protein and, accord-ingly, the Brh2-Dss1 complex is functionalas a recombination mediator with humanRad51 (71).

Owing to its uncommonly large size, full-length human BRCA2 protein has not yetbeen puried. To circumvent this problem,San Filippo et al. (72) fused the BRC3 andBRC4 repeats of BRCA2 to the DNA bindingdomain of this protein and devised a methodto purify the BRCA2-derived polypeptide(termed BRC3/4-DBD) to near homogene-ity. BRC3/4-DBD binds both ssDNA anddsDNA but with a distinct preference forthe former. As expected, BRC3/4-DBD in-teracts with Rad51 but not RecA. A quantityof BRC3/4-DBD substoichiometric to Rad51promotes the assembly of the Rad51 presy-naptic lament on RPA-coated ssDNA ef-ciently, but this BRCA2-derived polypep-tide is completely inactive toward RecA (72).Aside from its ability to restore presynapticlament assembly, BRC3/4-DBD also specif-ically targets Rad51 to ssDNA when an excessof dsDNA is present. Polypeptides that har-bor just the BRC3 and BRC4 repeats or theDBD, when used alone or in combination,do not exhibit recombination mediator orssDNA targeting activity, indicating that boththe BRC repeats and the DBD are neededfor biological efcacy and they act in cis (i.e.,both entities have to be present on the samepolypeptide). Thus, BRC3/4-DBD, and byinference BRCA2 protein, possesses two dis-tinct functional properties, i.e., an ability totarget Rad51 to ssDNA and a recombinationmediator activity, that are germane for thepromotion of Rad51-mediated HR reactions.That BRC3/4-DBD possesses recombinationmediator activity is congruent with a recentnding that fusion proteins comprising RPAand either a single BRC repeat or multiplesuch repeats can improve the HR prociencyof Brca2-decient hamster cells and enablethese cells to form Rad51 foci upon DNAdamaging treatment (129).

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A MODEL FOR THE BRCA2RECOMBINATION MEDIATORACTIVITY AND SOMEQUESTIONS

We present in Figure 3a model that incorpo-rates all the biochemical attributes believed tobe germane for the recombination mediatoractivity of BRCA2. Of special note is the coop-erative action of the BRC repeats and CTRBdomain in the assembly of the nascent Rad51lament, with the former acting as the initialdepositor of monomeric Rad51 onto the HRsubstrate and the latter being a recruiter of anintact Rad51 oligomer to seed the assembly ofthe nascent presynaptic lament.

As reviewed above, recent studies haveprovided evidence for functional differencesamong the BRC repeats of BRCA2 and possi-bly a presynaptic lament stabilizing activityin some (or all) of these repeats (122, 123).Future studies will address the contributionsof the individual BRC repeats toward presy-naptic lament assembly and preservation andthe mechanistic basis for their action. BRCA2is also able to interact with Dmc1 (115, 116),and studies in A. thaliana have provided di-rect evidence for functional interactions be-tween Brca2 and Dmc1 in meiotic HR (51).It seems reasonable to suggest that BRCA2also serves to nucleate Dmc1 presynaptic la-ment assembly on resected DSBs during mei-otic HR. Even though the CTRB domainof BRCA2 interacts with Rad51 more avidlythan Dmc1, the phosphorylation of S3291 inthis domain attenuates Rad51 binding with-out signicantly affecting the association withDmc1 (116). It will be interesting to testwhether CDK-mediated phosphorylation ofS3291 in BRCA2 provides a means for thedifferential regulation of BRCA2-Rad51 andBRCA2-Dmc1 interactions.

THE BRCA2-ASSOCIATEDPROTEINS DSS1 ANDPALB2 (FANCN)

BRCA2 forms a complex with the small acidicprotein DSS1 (130), which is needed for HR

+

b

Rad51 RPA

BRC-Rad51

CTRB-Rad51

BRCA2

Key:

or

Rad51 Rad51+ BRCA2

c

BRCA2

PALB2RPA

Rad51 Rad51Dmc1

BRC repeats DBD

Dmc1 DSS1

Interactiondomain:

a

HD1 2 3 4 5 6 7 8

OB

1

OB

2

OB

3

CTRB

Figure 3BRCA2 and its role in homologous recombination. (a) The functionaldomains in human BRCA2 are indicated. (b) In this model for the BRCA2recombination mediator activity, the BRC repeats each bind a Rad51monomer (for simplicity, only two of the six functional BRC repeats aredepicted as being associated with Rad51), while the CTRB is charged withan intact Rad51 heptamer. One of the BRC repeats and the CTRB thencooperate to assemble a nascent Rad51 lament, followed by the furtherdisplacement of RPA by the growing Rad51 lament. (c) Recent results (72)suggest a function of BRCA2 in targeting Rad51 specically to the ssDNAregion of a resected DSB.

efciency in vivo (131, 132). In U. maydis, theDss1 orthologue is thought to maintain Brh2in an active state by preventing the forma-tion of Brh2 homo-oligomers (128). Whethermammalian DSS1 serves a similar role has not

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yet been explored. Since the DSS1 interactionsurface resides within the DNA binding do-main of BRCA2 (79), DSS1 may well modu-late the DNA binding properties of the latter.Even though a homologue of DSS1 (calledSem1) is present in the budding yeast, it ap-pears to affect diverse processes such as exocy-tosis (133) and DSB repair by HR and NHEJ(134) via an association with the proteasome(134, 135).

A novel BRCA2-associated protein calledPALB2 was recently identied by Xia et al.(136). PALB2 promotes the proper localiza-tion and maintains the stability of BRCA2 inchromatin and nuclear matrix and appears tobe critical for the DNA repair and checkpointfunctions of BRCA2 (136). The PALB2 in-teraction domains lies within the amino ter-minus of BRCA2, and the cancer-associatedY42C mutation that affects the interaction ofBRCA2 with RPA (83) also ablates PALB2binding (136). PALB2 possesses a series ofWD40 repeats at its carboxyl terminus (136,137) that are indispensable for complex for-mation with BRCA2 and the biological ac-tivity of PALB2 (137). Importantly, muta-tions in PALB2 are associated with familialbreast cancer (138, 139) and biallelic inactiva-tion of PALB2 can cause Fanconi anemia (FA)of complementation group N (140, 141) (seesidebar). Owing to its FA connection, PALB2is also known as FANCN. How the HR func-tion and specically how the recombinationmediator activity of BRCA2 may be inu-enced by PALB2 will undoubtedly be a hotlypursued area of research in the near future.

THE S. CEREVISIAERAD55-RAD57 COMPLEXAND ITS RECOMBINATIONMEDIATOR ACTIVITY

Mutants of RAD55 and RAD57 genes sharethe uncommon property of being coldsensitive for HR and DNA repair (19, 142),and both genes are required for the deliveryof Rad51 to HR substrates in cells (73, 76,77). Overexpression of Rad51 or Rad52

partially overcomes the DNA repair and HRdecits of rad55 and rad57 mutant cells, andsimultaneous overexpression of Rad51 andRad52 leads to further suppression of themutant phenotype (143, 144). The RAD55-and RAD57-encoded proteins are regardedas paralogues of Rad51 as they exhibitsignicant homology to Rad51, includingsequence motifs that are thought to conferthe ability to bind and hydrolyze ATP (19,145). Rad55 protein is phosphorylated in acheckpoint-dependent manner when DNAdamage occurs, and this modication appearsto be important for Rad55 function upongenome-wide genotoxic stress (146, 147).Interactions between Rad55 and Rad57 andof Rad55 with Rad51 have been noted inthe yeast two-hybrid system (143, 144). Themajority or all of the Rad55 and Rad57proteins in yeast cell extract can be coim-munoprecipitated, indicating that they areassociated as a stable complex in cells (148).When coexpressed in yeast cells, the Rad55and Rad57 proteins assemble into a het-erodimeric complex that has ssDNA bindingactivity and the ability to interact with Rad51(148; P. Sung, unpublished results). Eventhough results from a genetic study suggestthat biological activity of the Rad55 proteinis inuenced by ATP (144), the Rad55-Rad57complex does not seem to have a signi-cant ATPase activity and its DNA bindingfunction appears to be refractory to ATP(P. Sung, unpublished results). Despite thesimilarity of both Rad55 and Rad57 toRad51, the Rad55-Rad57 complex has norecombinase activity (148). Addition of anamount of the puried Rad55-Rad57 complexsubstoichiometric to Rad51 overcomes theinhibitory effect of RPA on Rad51-mediatedhomologous DNA pairing and strand ex-change (148), indicative of a recombinationmediator activity. As deduced from the anal-ysis of a RAD51 allele (RAD51 I345T ) thataffords partial suppression of the rad55 andrad57 mutant phenotype, the Rad55-Rad57complex possibly also stabilizes the alreadyassembled Rad51 presynaptic lament (149).

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Much remains to be learned about the HRfunction of the Rad55-Rad57 complex. Mostnotably, whether the recombination mediatoractivity of Rad55-Rad57 involves a specic in-teraction with RPA, as has been demonstratedfor Rad52, is not yet known. The role, if any,of Rad55 phosphorylation and of ATP in themodulation of the recombination mediatorfunction of Rad55-Rad57 are also importantquestions that need to be answered.

THE HUMAN RAD51B-RAD51CCOMPLEX AND ITSRECOMBINATIONMEDIATOR ACTIVITY

The RAD51B and RAD51C genes code fortwo of the ve Rad51 paralogues (the remain-ing three Rad51 paralogue-encoding genesare RAD51D, XRCC2, and XRCC3) in verte-brate cells (19). There is good evidence thatall the Rad51 paralogues participate in HR byinuencing the assembly and/or maintenanceof the Rad51 presynaptic lament (19, 150),and the overexpression of human Rad51 pro-tein suppresses the defects of chicken DT40cells that lack any one of the ve Rad51 par-alogues (151). The Rad51B and Rad51C pro-teins interact in the yeast two-hybrid system(152) and form a stable complex as evidencedby coimmunoprecipitation and other means(153, 154). The Rad51B-Rad51C complexhas been puried to near homogeneity andfound to possess ssDNA binding activity anda modest ATPase activity that is stimulatedby DNA, ssDNA in particular, but the DNAbinding activity of this complex seems to berefractory to ATP (154). The DNA bindingand ATPase activities of the Rad51B-Rad51Ccomplex are derived from both Rad51B andRad51C proteins (155). Biochemical experi-ments (154) revealed a recombination media-tor activity in the Rad51B-Rad51C complex.Like other recombination mediators that havebeen characterized to date, Rad51B-Rad51Cacts in a catalytic fashion in that optimal re-combination mediator activity requires a rel-atively small amount of the complex (154).

Rad51B-Rad51C enhances the homologousDNA pairing activity of Rad51 (154), aneffect that could stem from the ability ofRad51C to promote the melting of duplexDNA (155). Whether Rad51B-Rad51C inter-acts with RPA in nucleating Rad51 presynap-tic lament assembly and if ATP binding andhydrolysis by the protein complex inuenceits recombination mediator function are stillunanswered.

Yeast two-hybrid and biochemical studieshave found a higher-order complex consist-ing of the Rad51B, Rad51C, Rad51D, andXRCC2 proteins, termed the BCDX2 com-plex (153). The BCDX2 complex has thehighest afnity for branched DNA (156) andcan recognize nicks in DNA (153). Rad51Dand XRCC2 on their own form a het-erodimeric complex that has DNA bindingactivity (157) but apparently no recombina-tion mediator activity (W. Bussen & P. Sung,unpublished results). It will be particularly in-teresting to test whether the BCDX2 complexhas enhanced recombination mediator activ-ity compared to the Rad51B-Rad51C com-plex. Rad51C also combines with the XRCC3protein to form a distinct protein complex thatseems to be associated with a nuclease activitycapable of resolving the HJ (158). This lat-ter observation is consistent with an apparentlate role of the XRCC3 protein in HR (159)and the fact that, in chicken DT40 cells, si-multaneously ablating the Xrcc3 and Rad51Dgenes engenders a mutant phenotype more se-vere than that of the respective single mutants(150).

THE S. POMBE SWI5-SFR1COMPLEX AND ITSRECOMBINATIONMEDIATOR ACTIVITY

As determined by a combination of yeast two-hybrid, coimmunoprecipitation, and geneticanalyses, S. pombe Swi5 protein combines witheither Swi6 or Sfr1 to form two separate com-plexes that have distinct functions. Specif-ically, the Swi5-Swi6 complex plays a role

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in mating-type switching (160), whereas theSfr1-Swi5 complex is needed for mitotic andmeiotic HR (64, 160). The sfr1 and swi5 nullmutants are partially impaired for the abil-ity to assemble DNA damageinduced fociof Rph51 (which is the S. pombe Rad51 or-thologue), and the DNA repair defects ofthese mutant cells can be partially suppressedby the overexpression of Rhp51. The Sfr1-Swi5 complex appears to provide a functionin HR similar to that of the Rhp55-Rhp57complex (which is orthologous to the S. cere-visiae recombination mediator Rad55-Rad57complex), as swi5, rph57 double mutant cellsare more severely HR impaired and decientin DNA damageinduced Rph51 focus for-mation than the single mutants (161). Takentogether, the genetic and cytological observa-tions suggest that Swi5-Sfr1 regulates Rph51presynaptic lament assembly and/or main-tenance, and that it acts independently of theRph55-Rph57 complex in this regard (161).

The Sfr1-Swi5 complex (which harborsone Sfr1 molecule and two Swi5 molecules)has been expressed in E. coli, puried, andcharacterized by Haruta et al. (64). The Sfr1-Swi5 complex physically interacts with bothRph51 and Dmc1 through Sfr1 (64, 160).Sfr1-Swi5 enhances the homologous DNApairing and strand exchange activity of Rph51and Dmc1 and can function in conjunctionwith both recombinases in the displacementof RPA from ssDNA. Thus, the biochem-ical analyses of Haruta et al. (64) reveal arecombination mediator activity in the Sfr1-Swi5 complex and also an ability of this com-plex to stimulate the homologous DNA pair-ing and strand exchange potential of the tworecombinases.

RELATIONSHIP OF THES. CEREVISIAE MEI5-SAE3COMPLEX TO THE S. POMBESWI5-SFR1 COMPLEX

The S. cerevisiae MEI5- and SAE3-encodedproteins are structurally related to the S. pombeSfr1 and Swi5 proteins, respectively (74). Un-

like their S. pombe counterpart, the expres-sion of Mei5 and Sae3 proteins is restricted tomeiosis (74, 162). Deletion of MEI5 or SAE3impairs meiotic HR and the ability to mountnuclear Dmc1 foci in response to meiotic DSBformation. Taken together, the mutant and cy-tological analyses provide evidence that Mei5,Sae3, and Dmc1 proteins operate in the samerecombination pathway and suggest a criti-cal role of Mei5 and Sae3 in the delivery ofDmc1 to the HR substrate (74, 162). Mei5and Sae3 proteins form a complex that phys-ically interacts with Dmc1 (74). Consideringwhat is known about the functional proper-ties of the S. pombe Sfr1-Swi5 complex (64),it will be particularly relevant to test whetherthe Mei5-Sae3 complex enhances the recom-binase activity of Dmc1 and Rad51 and pro-vides a recombination mediator activity forthe two recombinases.

BIPARTITE ACTION OF THEHOP2-MND1 COMPLEX INRECOMBINASEENHANCEMENT

That HOP2 and MND1 genes are critical formeiotic recombination was demonstrated ingenetic studies in S. cerevisiae and mice (163169). Based on extensive genetic analyses in S.cerevisiae, it has been deduced that the Hop2and Mnd1 proteins function with Rad51 andDmc1 to ensure the timely formation of DNAintermediates critical for the completion ofmeiotic recombination (163165, 167, 168).Although the expression of HOP2 and MND1is restricted to meiosis in S. cerevisiae, thesegenes are also expressed in somatic tissuesin plants, mice, and humans (165, 169171).This latter observation hints at the possibil-ity that in higher eukaryotes, the HOP2- andMND1-encoded products inuence mitoticHR as well.

The Hop2 and Mnd1 proteins can becoimmunoprecipitated from meiotic S. cere-visiae cell extract, indicating that they exist ina complex (167). Consistent with this nding,when coexpressed in E. coli, Hop2 and Mnd1

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proteins assemble into a stable, heterodimericcomplex (163, 172, 173). The Hop2-Mnd1complex binds dsDNA preferentially over ss-DNA (42, 163, 173) and appears to have aneven higher afnity for branched DNA (172).Studies using puried components revealedthat the mouse Hop2-Mnd1 complex directlyinteracts with Rad51 and Dmc1 (174) but notwith E. coli RecA (42). Although Hop2 andMnd1 proteins can individually bind DNAand interact with Rad51 and Dmc1 (42, 173),Hop2 has much higher afnity for DNA andMnd1 possesses greater avidity for Rad51(42). The Hop2-Mnd1 complex from mam-malian and yeast species strongly stimulatesthe recombinase activity of Dmc1 (163, 172,174), and the mouse and human Hop2-Mnd1complexes are just as active toward Rad51 inthis regard (172, 174). Because Hop2-Mnd1does not enhance the recombinase activity ofthe E. coli RecA protein (42), physical associa-tion of Hop2-Mnd1 with Rad51 and Dmc1 islikely important for functional interaction tooccur.

Recent studies by Chi et al. (42) andPezza et al. (43) have revealed that recom-binase enhancement afforded by the Hop2-Mnd1 complex occurs at two critical stagesof the homologous DNA pairing reaction.First, Hop2-Mnd1 stabilizes the presynapticlament of Rad51 and Dmc1, as shown us-ing a variety of approaches (42, 43) when thepresynaptic lament is rendered stable by theuse of a nonhydrolyzable ATP analogue, cal-cium ion, or the Rad51 K133R protein (whichbinds but does not hydrolyze ATP), Hop2-Mnd1 still exerts a strong stimulatory effect onDNA joint formation, leading to the deduc-tion that it must also act at a stage subsequentto presynaptic lament assembly. Importantly,Chi et al. and Pezza et al. have shown that theHop2-Mnd1 complex works in conjunctionwith the presynaptic lament to capture du-plex DNA molecule to facilitate the assemblyof the synaptic complex (42, 43; P. Chi & P.Sung, unpublished data). Duplex capture byHop2-Mnd1 and Rad51 or Dmc1 is not de-

pendent on homology in the incoming duplexmolecule (42, 43) but requires a functionalpresynaptic lament (42). Thus, Hop2-Mnd1acts in a bipartite fashion in Rad51/Dmc1-mediated homologous DNA pairing: stabi-lization of the presynaptic lament and du-plex capture to enhance synaptic complexformation.

Figure 4 presents a model that depicts thebipartite action of the Hop2-Mnd1complexin its enhancement of Rad51 and Dmc1activity. Future studies will determine therelative importance of the presynaptic la-ment stabilization and duplex capture rolesof Hop2-Mnd1 in HR reactions. Since theHop2-Mnd1 complex appears to have ahigh afnity for branched DNA structures(172; J. San Filippo & P. Sung, unpublishedresults), it remains possible that Hop2-Mnd1recognizes and stabilizes the nascent DNAloop formed by the two recombinases.

THE MULTIFUNCTIONAL ROLEOF THE DNA MOTOR PROTEINRAD54 IN HR

Rad54 is a member of the Swi2/Snf2 super-family of proteins and, similar to other mem-bers of that family, has dsDNA-dependentATPase, DNA translocase, DNA supercoilingand chromatin remodeling activities. Recentreviews (175, 176) have summarized someof the roles of this multifunctional factor inHR. Notably, Rad54 interacts with Rad51and is required at multiple stages in HR,in the early stages to promote a search forDNA homology, chromatin remodeling, andD-loop formation, and in the postsynapticstage to catalyze the removal of Rad51 pro-tein from dsDNA. The ability of Rad54 toremove Rad51 from dsDNA is believed toprevent the nonspecic association of Rad51with bulk chromatin and to provide DNApolymerases access to the 3-OH primer ter-minus in the nascent D loop to initiate therepair DNA synthesis reaction (175). Rad54also mediates the ATP hydrolysis-driven

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Hop2-Mnd1

Rad51 alone

D-loop formationdisfavored

Presynaptic filamentstabilization

Synaptic complex formation

D-loop formation

Hop2

Mnd1

Rad51

Key:

Figure 4The bipartiteaction of theHop2-Mnd1complex inrecombinaseenhancement.Hop2-Mnd1 actsin two critical stepsto enhance therecombinaseactivity of Rad51.Hop2-Mnd1 rststabilizes thepresynapticlament and thencooperates with thepresynapticlament to capturea dsDNA molecule(42). Hop2-Mnd1also functionallyinteracts withDmc1 in the samefashion (43).

migration of branched DNAs including theHJ and acts in conjunction with Rad51 to pro-mote a DNA strand exchange reaction that in-volves two duplex molecules (47). Under cer-tain in vitro conditions, Rad54 can dissociatethe D-loop intermediate, an activity thoughtto be relevant for the promotion of SDSA(177). By ChIP, the synapsis of the MAT ini-tiator and HML donor sequences can be de-tected in the absence of Rad54 (76). Whetherthis reects a role of Rad54 in plectonemicDNA joint formation or the initiation of re-pair DNA synthesis remains to be established(76).

Rad51 and Rad54 enhance each others ac-tivities (47, 175). Given the multifaceted role

of Rad54 in HR, one might expect RAD54to be essential to HR and DSB repair. Whileyeast rad54 mutants are extremely sensitiveto ionizing radiation and other types of DNAdamage that induce DSBs and are severely re-duced in spontaneous and induced mitotic re-combination (2, 3), rad54 mutants are amongthe least debilitated in meiosis of the HR mu-tants. In contrast to rad51 and rad52 mutants,rad54 mutants are able to form viable meioticproducts, although with reduced efciencycompared to wild type (178, 179). The lackof a strong meiotic phenotype of S. cerevisiaerad54 mutant cells can be attributed to theRad54-related protein Rdh54 (179; see be-low). Nonetheless, overexpression of Rad54

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can suppress the meiotic defects caused by thedmc1 mutation (180). In mouse ES cells, lossof Rad54 results in a slightly reduced HR fre-quency, sensitivity to ionizing radiation andmitomycin C, and aberrant repair of DNAdamage, whereas rad54/ mice appear to benormal (183, 184). This stands in contrast toother mammalian HR genes such as RAD51,as loss of Rad51 in mouse ES cells is lethal andthe mouse rad51/ genotype is an embryoniclethal (6, 185).

The stronger mitotic phenotype of rad54mutants in yeast and mammals may reecta preferential action of Rad54 in promotingDSB repair between sister chromatids. In-deed, genetic studies in S. cerevisiae have sug-gested that Rad54 acts in sister chromatidrecombination (186). The modest knockoutphenotype in vertebrates may reect the pre-dominant use of nonhomologous end joining(NHEJ) to repair DSBs. This suggestionis supported by the nding that chromo-some loss and rearrangements are increased inrad54 mutants in yeast and mammalian cells,and that loss of the NHEJ pathway in combi-nation with a rad54 mutation has a synergis-tic effect on chromosome instability, DNA-damage sensitivity, and cell growth (187). Thisshows that mammalian Rad54 is critical forDSB repair and for maintenance of genomicstability. Several hRAD54 mutations that re-duce or eliminate Rad54 activity in vitro havebeen found in human tumors, suggesting animportant role of Rad54 in cancer avoidance(188191).

RAD54-RELATED DNA MOTORPROTEINS: S. CEREVISIAERDH54 AND HUMAN RAD54B

RAD54 paralogues exist in both yeast andmammals. The mammalian paralogue iscalled RAD54B. The Rad54B protein has bio-chemical activities that are similar to thoseof Rad54 (191193). Rad54B interacts withRad51, has a dsDNA-dependent ATPase ac-tivity, and can translocate on duplex DNA to

result in topological changes in the DNA andthe transient opening of the DNA strands.Similar to Rad54, Rad51 enhances the activi-ties of Rad54B, and Rad54B promotes D-loopformation by Rad51 (193). The recombinaseactivity of Dmc1 is also stimulated by Rad54B(55, 194).

Mouse ES cells decient in Rad54Bhave no overt HR defect as measured bygene-targeting frequencies, but Rad54/

Rad54B/ cells are decreased in gene-targeting efciency below that of theRad54/ single mutant, showing thatRad54B functions in HR and this is revealedonly when Rad54 is absent (193). Studies ofionizing radiation and mitomycin C sensitiv-ities showed that mouse Rad54B has a rolein repairing DNA damage caused by theseagents. Mice that are decient in both Rad54and Rad54B are very sensitive to mitomycinC treatment, with particular damage to thebone marrow. While Rad54/ mice showsome abnormalities in meiotic chromo-some structure, Rad54/, Rad54B/, andRad54/ Rad54B/ mice are fertile (193).

S. cerevisiae also harbors a RAD54 par-alogue, called RDH54 or TID1 (179, 195).However, based on mutant phenotypes, par-ticularly the meiotic mutant phenotype,RDH54 is not equivalent to RAD54B. Rdh54protein has functional attributes similar tothose of Rad54, including dsDNA-dependentATPase, DNA translocase, DNA branch mi-gration, and DNA supercoiling activities,and also the ability to enhance the Rad51-mediated D-loop reaction (196199). It alsocan remove Rad51 from duplex DNA in anATP hydrolysis-dependent fashion (196), anactivity that may become important in thelater stages of HR (196), and in the adaptationfrom a DSB-induced checkpoint arrest (200).Rdh54 seems to be able to remove Dmc1 fromnonrecombinogenic chromatin sites as well(201; P. Chi & P. Sung, unpublished results).

Rdh54 interacts with Dmc1 (202). Al-though the meiotic role of Rdh54 is notfully understood, rdh54 mutants are severely

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reduced in meiotic viability and the doublemutant rad54 rdh54 fails to form viable mei-otic progeny or repair meiotic DSBs (179,203). The mitotic phenotype of the dou-ble mutant is most visible in diploid cells,seen as a failure to grow uniformly owingto spontaneous DNA damage and check-point arrest. rdh54 mutants are also defec-tive in resuming growth after a DSB-inducedcheckpoint arrest (200). These ndings indi-cate that aside from some functional over-lap, RAD54 and RDH54 have independentfunctions in HR, DSB repair, and otherprocesses.

Genetic studies by several groups havesuggested that in meiosis Rad54 may havea prominent role in promoting DSB re-pair through HR between sister chromatids,whereas Rdh54 is required for interhomo-logue HR (180182, 186). How this distinc-tion is made at the molecular level is notknown.

While Rad54 and Rdh54 can move Rad51bound to duplex DNA and can remodel chro-matin in vitro, in vivo other DNA or chro-matin binding proteins may well be targetsof these proteins. A recent study (204) hasfound that the meiotic-specic sister chro-matid cohesion factor Rec8 and the mitoticsister chromatid cohesion factor Mcd1/Scc1shows aberrant distribution on chromosomesin mutant rdh54 meioses. Chromosomes aremis-segregated in both of the meiotic divi-sions, which is attributed to a failure of sis-ter chromatid separation. Whether Rdh54acts directly to remodel cohesin throughMcd1/Scc1 and Rec8 is not known. How-ever, loading of cohesin at DSBs is criticalfor repair of mitotic DSBs (205208), so itis conceivable that cohesin loading and re-modeling at meiotic DSBs by Rdh54 is criti-cal for proper meiotic interhomologue HR.Mcd1/Scc1association on chromatin duringmeiosis in the absence of Rdh54 appears tobe the cause of chromosome mis-segregation.One intriguing possibility is that Mcd1/Scc1-mediated cohesion is important in distin-guishing sister from nonsister chromatids in

HR. If true, then the deciency of rdh54 mu-tant cells in interhomologue HR could arisefrom an inability of nonsister chromatids tointeract due to persistent cohesion of the sisterchromatids.

CONCLUSIONS

Defects in HR cause genomic instability.When the instability leads to aberrant ex-pression or regulation of tumor suppressorsor oncogenes, cell transformation and cancermay ensue. Because HR can give rise to alter-ations in the genomic conguration, it mustbe nely controlled to avoid deleterious chro-mosome rearrangements and the generationof pathological intermediates (8, 9).

As detailed here and elsewhere (8, 9,33), the basic HR machinery and its mech-anism and regulation are remarkably con-served among eukaryotes. The HR reactionmediated by either Rad51 or Dmc1 has atleast two rate-limiting steps, assembly of thepresynaptic lament and capture of duplexDNA by the presynaptic lament. As has beenreviewed in this article, recent biochemicalstudies have unveiled recombinase accessoryfactors that function to overcome these rate-limiting steps.

One of the most exciting developments inunderstanding HR mechanism and its healthrelevance is the identication of the tumorsuppressor BRCA2 as a recombination me-diator. Aside from interactions with Rad51,Dmc1, RPA, and DNA, additional domainswithin BRCA2 mediate its association withother factors, such as PALB2, that are im-portant for its biological functions. Whetherthese factors inuence the recombination me-diator activity of BRCA2 is not yet known.Moreover, how the BRCA2-PALB2 complexfunctionally links the HR machinery to the FApathway of DNA-damage repair and responseremains to be delineated.

Studies on the Swi2/Snf2-like DNA mo-tor proteins Rad54, Rdh54 and Rad54B havebegun to elucidate their multifaceted role inHR. Rad54 and Rdh54 help determine the

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selection of the sister chromatid or nonsis-ter chromatid as HR substrate. The geneticconsequences of recombination between thesister or nonsister chromatids have a profound

effect on sequence variation and meiotic chro-mosome segregation. How this distinction ismade or regulated will be a particularly inter-esting problem to tackle.

SUMMARY POINTS

1. Homologous recombination is required for DNA double-strand break repair, repairof damaged replication forks, repair of incomplete telomeres, and correct segregationof homologous chromosomes in meiosis.

2. Homologous recombination is critical for suppression of genome instability and tumorformation.

3. Homologous recombination is mediated by recombinases, a conserved group of pro-teins from bacteria to humans. The eukaryotic recombinases, orthologues of E. coliRecA, are Rad51 and Dmc1.

4. Rad51 and Dmc1 mediate the homologous DNA pairing and strand exchange reactionthrough the presynaptic lament, single-stranded DNA coated with Rad51 or Dmc1.

5. Presynaptic lament assembly is slow and prone to interference by the single-strandedDNA binding protein RPA and so requires the involvement of recombination medi-ator proteins, such as BRCA2, for enhancement.

6. Synaptic complex formation by Rad51 and Dmc1 are enhanced by the Hop2-Mnd1complex, revealing that capture of homologous duplex DNA is a rate-limiting step inhomologous recombination.

7. Several DNA motor proteins, such as Rad54 and Rdh54, function at several steps inhomologous recombination. They promote homologous pairing and strand exchangeon naked DNA and chromatinized DNA an