JOUINAL OF BACTERIOLOGY, May 1981, p. 684-6910021-9193/81/050684-08$02.00/0

Vol. 146, No. 2

a/a-Specific Effect of the mms3 Mutation on UltravioletMutagenesis in Saccharomyces cerevisiaePATRICIA MARTIN,' LOUISE PRAKASH,l* AND SATYA PRAKASH2

Department ofRadiation Biology and Biophysics, University ofRochester School ofMedicine andDentistry, Rochester, New York 14642,' and Department ofBiology, University ofRochester, Rochester,

New York 146272

Received 20 November 1980/Accepted 21 February 1981

A new gene involved in error-prone repair of ultraviolet (UV) damage has beenidentified in Saccharomyces cerevisiae by the mms3-1 mutation. UV-inducedreversion is reduced in diploids that are homozygous for mms3-1, only if they arealso heterozygous (MATa/MATa) at the mating type locus. The mms3-1 muta-tion has no effect on UV-induced reversion either in haploids or MATa/MATaor MALTa/MATa diploids. The mutation confers sensitivity to UV and methylmethane sulfonate in both haploids and diploids. Even though mutation inductionby UV is restored to wild-type levels in MATa/MATa mms3-1/mms3-1 orMATa/MATa mms3-1/mms3-1 diploids, such strains still retain sensitivity to thelethal effects of UV. Survival after UV irradiation in mms3-1 rad double mutantcombinations indicates that mms3-1 is epistatic to rad6-1 whereas non-epistaticinteractions are observed with rad3 and rad52 mutants. When present in thehomozygous state inMATa/MATa hisl-ll hisl-315 heteroallelic diploids, mns3-1 was found to lower UV-induced mitotic recombination.

Three epistasis groups exist in the yeast Sac-charomyces cerevisiae for the repair of ultravi-olet (UV) light-induced damage in DNA. Onegroup consists of nine genes (RAD1, RAD2,RAD3, RAD4, RAD7, RAD10, RAD14, RAD16,and MMS19) involved in pyrimidine dimer re-moval (18). Another group contains eight genes(RAD50 to RAD57) which, when mutant, confersensitivity primarily to ionizing radiation (6) andare somewhat UV sensitive. Several genes of thethird epistasis group, such as RAD6, REVI,REV2, and REV3 play a role in error-pronerepair of UV damage. Mutations in the RAD6and REV3 loci, however, result in a substantialdecrease in UV mutability at all loci tested andare therefore general in their effect (10, 13, 14).Strains with mutations at these two loci wereobtained by different screening methods: rad6-1 by screening for sensitivity to UV (2), and rev3by screening for strains showing reduced UV-induced reversion of the highly UV-revertibleallele arg4-17 (13). Mutants of the RAD6 locusalso show greatly reduced mutations induced bythe alkylating agents ethyl methane sulfonateand N-methyl-N'-nitro-N-nitrosoguanidine (15).Mutants of the REV3 locus, on the other hand,have no effect on ethyl methane sulfonate-in-duced mutations.

In this study, we report on another locus,MMS3, which also affects UV induced muta-tions: the mms3-1 mutant, isolated by screening

for sensitivity to methyl methane sulfonate, af-fects UV-induced mutations at all loci tested;however, reduced UV-induced mutations are ob-served only in MATa/MATa diploids. Diploidsthat are homozygous for the mating type locusand homozygous for mms3-1, or haploids of a ora mating type that carry the mms3-1 mutation,all show UV mutability characteristic ofMATa/MATa, MATa/MATa, MATa/MATa, MMS+/MMS+, or MMS+/mms3-1 diploids, or MMS+haploids.

(Part of this work was submitted by P.M. inpartial satisfaction of the requirements for thePh.D. degree, University of Rochester, Roches-ter, N.Y.)

MATERIALS AND METHODSStrains. The mutant strain MD-3 (MATa cycl-115

h81-1 lys2-1 trp2 tyr phe mm8i) was isolated fromstrain B-635 (MATa cycl-115 hisl-1 lys2-1 tip2MMS+) among clones unable to grow on mediumcontaining methyl methane sulfonate (17). The mu-tation in the strain MD-3, designated mms3-1, com-plements radl through rad22, rad5O through rad57,revl, rev2, and rev3. The origin of the other mutationsin this study is as follows: rad3-2 (uvsl3) and rad6-1(uvs6) from B. S. Cox; rad9-4 (rs2) from F. Eckardt;rad9-6 from R. Snow and rad18-2 (uxsl) and rad52-1(xsl) from M. A. Resnick. The arg4-17 and lys2-1alleles are ochre suppressible nonsense alleles (8), andhisl-1, hisl-7, and hisl-315 are not suppressible (23).Standard yeast genetic techniques were used to derive

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strains with particular genotypes required for variousexperiments.

Media. The standard yeast culture media yeastextract-peptone-dextrose (YPD), YPD + 0.035%methyl methane sulfonate, synthetic dextrose, syn-thetic complete (SC), and omission media (SC lackingone nutrient) were as previously described (15, 17).YPD + 1 ,M cryptopleurine (YPD + CRY) mediumwas made by adding CRY just before pouring theplates.

Chemicals. Methyl methane sulfonate and ethylmethane sulfonate were obtained from Eastman Or-ganic Chemicals. They were purified by vacuum dis-tillation at 10 to 15 mm Hg pressure before use. CRYwas obtained from Chemasea Manufacturing Pty.,Ltd., Pearhurst, New South Wales, Australia.

Construction of homozygous mating type dip-loids. Resistance to CRY, a protein synthesis inhibi-tor, is due to a single recessive gene, cryl, located 2.1map units from the mating type locus on chromosomem (7, 22). Diploids homozygous for the mating typelocus were constructed by selecting for mitotic recom-binants generating CRY-resistant strains in CRY'l/crl heterozygotes. CRY-resistant colonies were ob-tained from PM65-2C, MATa arg4-17 lys2-1 tyrlmms3-1 CRY'l and PM65-1D, MATa adel arg4-17lys2-1 tyr phe mms3-1 CRY6l by plating on YPD +CRY. Colonies which grew on this media were picked,retested for resistance to CRY, and confirmed as hav-ing retained their original genotypes. PM65-2C cryland PM65-1D cryrl strains were then mated to arg4-17 CRY) strains and plated onto YPD + CRY platesto select for mitotic recombinants generating diploidshomozygous for cryl. CRY-resistant colonies (whichgrew up in 5 to 7 days at 30°C) arose at a much higherfrequency than would be expected via spontaneousmutation. Strains in which the crossover event in-cluded the mating type locus were obtained. The cri-teria used to indicate that a strain was a diploidhomozygous for both the mating type locus and cr/lwere: (i) the CRY-resistant colonies had the nutri-tional requirements (and only those requirements)expected of the diploid and not either haploid parent;(ii) when crossed to haploids, the putative homozygousmating type diploids mated only to what would beexpected if the crossover included both the CRY geneand the mating type locus, i.e., if one of the parentswas PM65-2C cryrl, MATa, the CRY-resistant diploidderived from it would only mate with MATa haploids;(iii) when the putative homozygous mating type dip-loid, which was also homozygous wild type (+/+) fora particular nutritional marker, was mated to a haploidcontaining a recessive mutation for the same nutri-tional marker, that marker showed triploid, not diploidsegregation. Triploid segregation for the mating typelocus could also be monitored in these crosses. Al-though spore viability was rather poor, enough segre-gants survived to show triploid segregation.UV-induced reversion. Cells were grown to sta-

tionary phase in liquid YPD at 30°C. Cells were har-vested by centrifugation, washed once, and suspendedin water at a density of 5 x 107 cells per ml. Haploidstrains were sonicated to disperse clumps; as a rule,diploids did not require sonication. Cells from appro-priate dilutions were spread on omission media to

score for revertants or on SC medium to score forsurvivors. To avoid crowding artifacts, the maximumdensity plated was 107 cells per plate. The UV sourceand its dosimetry were as previously described (11).Plates were irradiated with covers removed at a flu-ence rate of 1 J/m2 per s if the total fluence was over10 J/m2 and 0.1 J/m2 per s if under 10 J/m2. Plateswere incubated in the dark to avoid photoreactivationfor 5 to 7 days at 30°C. Colonies were then counted.

Survival after UV irradiation. Cells were pre-pared as described for UV-induced reversion butplated on YPD rather than SC or omission media.After irradiation, plates were incubated for 3 days inthe dark at 30°C, and colonies were counted.UV-induced intragenic mitotic recombination.

The following protocol was used to reduce the spon-taneous level of prototrophs to a workable level:freshly grown haploid parents of the diploid to be usedwere mated on YPD, and diploids were obtained byprototrophic selection. A single diploid colony wasused to inoculate liquid medium of synthetic dextroseplus histidine plus other required amino acids and wasgrown for 3 days at 30°C. Such a procedure generallyresulted in initial prototroph frequencies of about 2 to300 per 107 cells. Cells were harvested and irradiatedas described for UV-induced reversion.

RESULTS

Sensitivity to UV light. mms3-1 confers amoderate amount of sensitivity to UV in hap-loids (Fig. 1A). The data points are the meansurvival of 23 mms3-1 and 27 MMSi strains.Similar results were obtained for diploid strainshomozygous for mms3-1 (Fig. 1B). MMSi/mms3-1 diploids were as resistant to UV asMMS+/MMSJ diploids; there was no evidenceof semidominance.UV-induced reversion. UV-induced rever-

sion of several different loci was examined instationary-phase haploid and diploid strainscarrying the mms3-1 mutation. Reversion oflys2-1, an ochre suppressible allele, was substan-tially decreased in mms3-1/mms3-1 comparedwith mms3-1/MMS+ and MMS+/MMS+ dip-loids (Fig. 2A), whereas there is only a marginaldifference, if any, between mms3-1 and MMS+haploids (Fig. 2B). The mms3-1 mutation alsodecreased reversion of another ochre suppressi-ble site, arg4-17. UV-induced reversion was low-ered about 10-fold in mms3-1/mms3-1 diploidsbut only about 2-fold in mms3-1 haploids (datanot shown). The mms3-2 mutation, an allele ofmms3-1, also showed a 10- to 20-fold reductionof UV-induced reversion of arg4-17 in mms3-2/mms3-2 diploids but not in mms3-2 haploids,where the reduction is only about 2-fold (Table1). The decreased UV-induced reversion ob-served in diploids homozygous for mms3-1 wasnot limited to ochre suppressible alleles. UV-induced reversion of the non-suppressible allelehisl-7 was greatly reduced in diploids (Table 2),

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L)MS °t mms 3=/

100O 10 20 30 40 50 0 O10 20 30 40 5

UV FLUENCE ( J/m2) UV FLUENCE (J/m2-)FIG. 1. Survival fluence response curves for haploid and diploid strains after UV irradiation. (A) Mean

survival of23 mms3-1 ( ) and 27MMS' (O) haploid -strains. fiB) Mean -survival of5 mms3-1I/mms93-1 ( ) and5MMS /mrns3-1 (O) diploid strains. Stationary cells irradiated on the sgurface ofSC or YPD pkttes showedsimnilar sensitivity to UV. Data points are the mean survival; error bars are the standard error of the mean.

but in mms3-1 haploids only about a twofoldreduction was observed (data not shown). Eventhough the mms3-1 mutation has a differentialeffect on Iys2-i-, arg4-17-, and hisl-7-inducedreversion when comparing haploids and diploids,mutant strains of both ploidy are sensitive tothe lethal effects of UV (Fig. 1). The reductionin UV-induced reversion which is dramatic indiploids but minor in haploids is due to themms3-1 mutation since several mutant strainswere found to behave similarly.Nature of UV-induced revertants. It has

been shown previously (13) and confirmed herethat most UV-induced revertants of arg4-17 areproduced by site reversions rather than by sup-pression. The presence of the mms3-1 gene doesnot seem to affect this process. Strains wereconstructed containing both lys2-1 and arg4-17ochre suppressible markers. Reversion experi-ments were carried out as described, but cellswere plated on both SC-lys and SC-arg media.Colonies were picked from SC-arg plates andtested for growth on SC-lys and vice versa.

Ninety-nine percent of the colonies grew only onone medium and not both, regardless ofwhetherthe strain was mms3-1 or MMS+.Effect of mating type on UV-induced re-

version in mm83-1/mm83-1 diploids. To de-termine ifthe reduction in UV-induced reversionin mms3-1/mmrs3-1 diploids is due to ploidy ormating type, diploids homozygous for the matingtype locus were constructed as described in Ma-

terials and Methods. UV-induced reversion ofarg4-17was determined in mms3-1 homozygotesand heterozygotes which were homozygous orheterozygous for the mating type locus (Fig. 3).The MATa/MATa mms3-1/mms3-1 strainswere the only diploids which showed substantialreduction in reversion frequencies. In theMATa/MATa or MATa/MATa mms3-1/mms3-1 diploids, UV-induced reversion frequen-cies were restored to the level observed inMMS'/mms3-1 strains homozygous or hetero-zygous for the mating type locus. Thus, themms3-1 mutation exerted its effect on UV-in-duced reversion mainly in MATa/MATa dip-loids. Zygosity at the mating type locus did notaffect reversion in MMS'/mms3-1 diploids (Fig.3).Even though diploids that are homozygous for

mms3-1 and the mating type locus, eitherMATa/MATa or MATa/MATa, show a differ-ent response to UV-induced reversion of arg4-17than do mms3-1/mms3-1 diploids that are het-erozygous for the mating type locus, the sensi-tivity to the lethal effects of UV was similar inmms3-1/mms3-1 strains that were MATa/MATa, MATa/MATa, or MATa/MATa (datanot shown). Similarly, the sensitivity ofMATa/MATa orMATa/MATa MMS+/mms3-1 strainswas the same as that ofMATa/MATa MMS+/mms3-1 diploids.UV-induced intragenic mitotic recombi-

nation. The effect of the mms3-1 mutation on

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I I I .I

10 20 30 40 50

1000I

100

10 I I

10 20 30 40

UV FLUENCE (J/m2) UV FLUENCE (J/m2)FIG. 2. Reversion of lys2-1 in diploid and haploid strains after UV irradiation. (A) Mean reversion in 5

mms3-1/mms3-1 (-),5 MMS+/mms3-1 (0) and I MMS /MMS+ (0) diploid strains. (B) Mean reversion in 4mms3-1 (0) and 5 MMS+ (0) haploid 8trains. Revertants at 0 Jlm' have been subtracted from inducedrevertants. Data points are mean revertants; error bars are standard error of the mean.

TABLE 1. UV-induced reversion of arg4-17 in haploid and diploid strains ofMMS+ and mms3-2UV fluence (J/m2)

Strain Genotype0 10 20 30 40

DiploidLP-2580 mms3-2/mms3-2 oa 4 9 22 21PM-495 MMS+/MMS+ 6 33 191 437 913

HaploidLP25724Db mms3-2 0 23 50 82 136LP2572-7Cb mms3-2 0 38 135 170 185PM651AC MMS+ 0 34 65 263 397PM65-1Cd MMS+ 5 27 103 278 529

aARG+ revertants per 107 survivors.bMATa and MA Ta haploids used to make the diploid LP-2580.MALTa haploid used to make the diploid PM-495.dMATa haploid from the same tetrad as PM65-1A.

UV-induced intragenic homologous mitotic re-combination was examined in diploid strains ofgenotype MATa/MATa hisl-l +/+ hisl-315.Neither hisl-l nor hisl-315 is very revertible byUV (Martin and Prakash, unpublished results);

hence we did not have to be concerned withmutational events complicating the recombina-tion data. At a fluence of 50 J/m2, the numberof histidine prototrophs arising is decreasedabout 10-fold in mms3-1/mms3-1 diploids com-

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TABLE 2. UV-induced reversion of hisl-7 in mms3-1/mms3-1, MMS+/mms3-1, and MMS+/MMS+

diploidsUV fluence (J/m2)

Strain0 10 20 30 40

mms3-1/mms3-1PM-489 Oa 14 20 50 61PM-500 0 1 5 7 5

MMS+/mms3-1PM-492 6 74 409 1,036 1,846PM-502 4 75 320 746 1,344

MMS+/MMS+PM-503 7 85 286 714 1,597a HIS+ revertants per 107 survivors.

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FIG. 3. ReveMATa mms3-1and 2 AATa XMMS+/MATamms3-1 (0); an1 MATa MM.after UV irradsubtracted frormean revertanmean.

pared to wildstrains, the fincreased wimms3-1 diplototrophs rosemitotic intral

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UV FLUENCE (J/m2)

FIG. 4. UV-induced mitotic recombination forstrains of genotype MATa/MATa hisi-I +/+ hisi-315 in three strains of mms3-1/mms3-1 (0), twostrains of MMS/mms3-1 (0), and two strains ofMMS/MMS+ (0). Data points are mean proto-trophs; error bars are standard error of the mean.

/it/ in strains of genotype hisl-l +/+ hisl-315, wasnot affected by the mms3-1 mutation. Simulta-neous lowering of both UV-induced mutationand UV-induced recombination suggests thatthe MMS3 function may be involved in both.Interactions with rad mutants. Since the

mms3-1 mutation has a generalized effect onlowering UV-induced reversion in MATa/MATa diploids, it was of interest to determinewhether it belongs to the same epistasis group

10 20 30 40 50 as rad6, which is involved in error-prone repairof UV damage. Interactions affecting UV sur-

UV FLUENCE (J/m2) vival of mms3-1 with rad3, a member of the

ersion of arg4-17 in 3 MATa mms3-1/ group of genes involved in excision of UV-in-

(0); 1MATamms3-1/MATamms3-1 duced pyrimidine dimers (16), with rad6, in-mms3-1 and I MATa MMS+(MATa volved in error prone repair, and with rad52, a

zd 1 MATa MMS;+/MATa mms3-1 and member of the third epistasis group of yeast (1),t/MATa mms3-1 (O) diploid strains which may be involved in recombinational re-!iation. Revertants at 0 JIm2 have been pair (19, 20), were determined. Results obtainedn induced revertants. Data points are for haploid segregants of a cross between mms3-ts; error bars are standard error ofthe 1 and rad3-2 are given in Fig. 5A. The rad3-2

haploids were much more sensitive than mms3-1 haploids; however, mms3-1 rad3-2 strains were

-type diploids (Fig. 4). In wild-type even more sensitive than rad3-2 strains. Similarrequency of histidine prototrophs results were obtained in mms3-1 rad52-1 strains.th fluence, whereas in mms3-1/ The mms3-1 rad52-1 haploids were more sensi-oids, the frequency of histidine pro- tive to UV than the more sensitive of the twomuch more slowly. Spontaneous single mutants, which in this case is the mms3-

genic recombination, as measured 1 haploid (Fig. 5B). These results indicate that

*- hisl-315

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mms3-1 and rad3-2 as well as mms3-1 andrad52-1 are in different epistasis groups for therepair of lethal damage induced by UV. In con-trast, mms3-1 rad6-1 double mutants were nomore sensitive to the lethal effects of UV lightthan the more sensitive of the single mutants, inthis case, rad6-1 (Fig. 5C). Such an interactionindicates that mms3-1 and rad6-1 belong in thesame epistasis group for the repair of UV dam-age.Effect of the RAD9 gene on UV-induced

mutation and recombination. Another mu-tant, rad9, was reported to affect UV-inducedmutation in diploids but not haploids (4). Wehave not been able to confirm previous reportsthat rad9-4 lowers UV-induced reversion of lys2-1 in diploids but not haploids. Diploids homo-zygous for rad9-4 or rad9-6 showed similar fre-quencies of Iys2-1 reversion as RAD+/rad9-4 orRAD+/RAD+ strains (Table 3). Induced rever-sion to lysine prototrophy in rad9-4 haploids islike that in RAD+ haploids (Table 3). The pub-lished work on UV mutation in the rad9-4 dip-loids was carried out in only one strain. Since nosegregation analysis was done, it is possible thatthe effect observed was not due to the rad9gene. We have not been able to confirm previousreports (9) that UV-induced intragenic mitoticrecombination is abolished in rad9-4 homozy-gous diploids. About the same number of histi-dine prototrophs arose at a fluence of 75 J/m2whether or not the cells were rad9-4/rad9-4 orRAD+/rad9-4 (Table 4).

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DISCUSSION

The mms3-1 mutation shows epistatic inter-actions for UV sensitivity only with rad6, amember of the group of genes involved in UVmutagenesis, and not with rad3, representativeof the group of genes involved in pyrimidinedimer excision, or with rad52, which affects ge-netic recombination. However, unlike the othermembers of the rad6 epistasis group which af-fect UV mutagenesis in both haploids and dip-loids, the effect of mms3-1 on UV-induced mu-tations is observed only in MATa/MATa dip-loids, even though it confers sensitivity to UV inboth haploids and diploids. For the ochre sup-pressible alleles arg4-17 and lys2-1, and the mis-sense allele hisl - 7, the frequency of UV-inducedrevertants was lowered to 5 to 70 times inMATa/MATa mms3-1/mms3-1 diploids com-pared with MATa/MATa MMS+/MMS+ dip-loids but only twofold in mms3-1 haploids com-pared with MMS+ haploids.

Several alternative hypotheses can be pro-posed to account for the behavior of MATa/MATa mms3-1/mms3-1 diploids compared withMATa/MATa mms3-1/mms3-1 and MATalMATa mms3-1/mms3-1 diploids and MATamms3-1 and MATa mms3-1 haploids: (i) theremay be an error-prone repair system for UVdamage in yeast which is dependent on hetero-zygosity at the mating type locus, and theMMS3gene may function in this error-prone repairsystem only in MATa/MATa diploids; (ii) the

UV FLUENCE (J/m2)FIG. 5. Survival after UV irradiation in mms3-1 rad mutants. Error bars are standard error of the mean.

(A) Mean survival of two MMS+ RAD' (0), two mms3-1 (0), two rad3-2 (0), and two mms3-1 rad3-2 (U)haploid strains. (B) Mean survival oftwo MMS+ RAD+ (0), two mms3-1 (-), two rad52-1 (C), and two mms3-I rad52-1 (U) haploid strains. (C) Mean survival of two MMS+ RAD' (0), two mms3-1 (0), two rad6-1 (0),and two mms3-1 rad6-1 (U) haploid strains.

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TABLE 3. UV-induced reversion of Iys2-1 in rad9 diploids and haploidsUV fluence (J/m2)

Strain0 10 20 30 40

Diploidrad9-4/rad9-4PM-433 ga 28 (100)b 130 (78) 209 (46) 319 (16)PM-435 16 41 (85) 128 (64) 316 (22) NcCLP-1695 53 82 (95) 293 (25) 466 (12) 1070 (2.0)

rad9-6/rad9-6LP-1819 0 8 (100) 73 (44) 283 (12.5) 474 (5.4)

RAD+/rad9-4PM-436 11 54 (93) 138 (95) 209 (99) 540 (81)

RAD+/RAD+LP-1729 3 NC 203 (100) 383 (95) 578 (90)PM-495 7 43 (90) 220 (92) 543 (93) 910 (93)

Haploidrad9-4N-231 11 34 (94) 101 (66) 255 (23) 587 (5.7)LP765-4B 42 93 (54) 173 (22) 312 (8.3) 642 (2.4)

RAD+PM391-4C 3 25 (95) 94 (80) 248 (73) 593 (54)

a LYS+ revertants per 107 survivors.b Percent survival in parentheses.C NC, Data not collected.

TABLE 4. UV-induced histidine prototrophs in hisl-) +/+ hisl)-315 strainsUV fluence (J/m2)

Strain0 25 50 75

PM-433 rad9-4/rad9-4 2"0a 4,440 (57)b 6,630 (5.4) 28,100 (0.35)PM-436 RAD+/rad9-4 109 5,139 (96) 18,109 (95) 37,709 (59)

a Histidine prototrophs per 107 survivors.b Percent survival in parentheses.

cell may contain another gene which can substi-tute for the MMS3 function. This other gene isturned off in MATa/MATa cells but can beexpressed in MATa/MATa, MATa/MATa,MATa orMATa cells; (iii) the mms3-1 mutationmay have an altered regulatory region whichnow responds to regulatory signals controllingmating functions. Mutations at several other lociare known which are expressed differentially inMATa, MATa, MATa/MATa and MATalMATa versus MATa/MATa cell types. Theseinclude mutations of ornithine transaminase(cargB+Oh), arginase (cargA+ oh) (3), the ureaamidolyase bienzymatic complex durOh (12)(which show constitutive synthesis in MATaand MATa haploids as well as MATa/MATaand MATa/MATa diploids but normal synthe-sis in MATa/MATa diploids), and the CYC7-H2 mutation (which causes an overproductionof iso-2-cytochrome c in MATa andMATa hap-

loids and MATa/MATa or MATa/MATa dip-loids but normal levels in strains heterozygousat the mating type locus, such asMATa/MATadiploids or MATa/MATa MATa/MATa tetra-ploids [21]). In the case of the CYC7-H2 muta-tion, it has been suggested that overproductionin haploids or MATa/MATa or MATa/MATadiploids is due to the insertion of a 5.5-kilobasesequence in the 5' nontranslating portion of thegene; this sequence is homologous to the trans-posable Tyl element of yeast (5). However, it isnot known whether mms3-1 or the three othermutations which show the mating type effect,cargB+Oh, cargA+O", and durO", also containan inserted Tyl element.

ACKNOWIED<GIENTSWe thank Beverly Errede for useful discussions.This paper is based on work performed partially under

Public Health Service research grant GM 19261 from theNational Institutes of Health and under contract number DE-

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AC02-76EV03490 with the U.S. Department of Energy at theUniversity of Rochester Department of Radiation Biology andBiophysics and has been assigned report no. UR-3490-1897.L.P. was supported in part by a Public Health Service CareerDevelopment Award (GM-00004) from the National Institutesof Health.

LITERATURE CITED1. Cox, B. S., and J. Game. 1974. Repair systems in Sac-

charomyces. Mutat. Res. 26:257-264.2. Cox, B. S., and J. M. Parry. 1968. The isolation, genetics

and survival characteristics of ultraviolet-light sensitivemutants in yeast. Mutat. Res. 6:37-55.

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