recombination homologous recombination site-specific recombination transposition different sets of...
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Recombination
• Homologous recombination
• Site-specific recombination
• Transposition
Different sets of proteins are responsible for thedifferent types of DNA exchange.
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Catalyzed by recombinases, serine or tyrosine recombinases.
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Insertion of the phage lambda genome into the E. coli genome.Catalyzed by sequence-specific recombinase (integrase) a special type I topoisomerase.
4600 kb
48 kb
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Site-specific recombination either integrates, deletes, or reversesa DNA sequence
Insertion Deletion(reversal of insertion)
Inversion
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Figure 15.4 Genomes 3 (© Garland Science 2007)
Mechanism of type I DNA topoisomerases
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Mechanisms of serine and tyrosine recombinases
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Chemistry of site-specific recombination
5’ phosphate remains covalently linked to the recombinase,as in reactions of other topoisomerases.
Reaction creates a single-strand break(nick). Reaction is reversible.
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Serine recombinases bound to DNA
Recombinases(dimers)
dsDNA
dsDNA
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Cre recombinases (tyrosinerecombinases) bound to aHolliday junction
Recombinases(dimers)
dsDNA
dsDNA
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Cre recombinases (tyrosinerecombinases)
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Figure 17.14 Genomes 3 (© Garland Science 2007)
Removal of the kanamycin resistance gene by induced expression of the Cre recombinase
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Example for insertion of DNA by site-specific recombination:Insertion of phage lambda genome into the E. coli genome
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Lytic and lysogenic lifecycles of phage lambda
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Example for insertion of DNA by site-specific recombination:Insertion of phage lambda genome into the E. coli genome
Integration requires a recombinase(called lambda integrase) and theintegration host factor (IHF) encodedby a gene in the E. coli genome.
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The integration host factor (IHF) bends the DNA to bring the cleavage sites together
IHF
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The recombination site in phage lambda
IHF
integrase
integrase
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Insertion of the phage lambda genome into the E. coli genome.Catalyzed by sequence-specific recombinase (integrase) a special type I topoisomerase.
attL
attR
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Binding of an “excisionase” to X sites in attR and attL brings the sites together for excision of the phage genome
IHF
integrase
integrase
Excisionase(encoded in the phage genome)
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Example for inversion of DNA by site-specific recombination:DNA inversion by the Hin recombinase of Salmonella
EncodesH2 flagellin
Encodes a repressorof H1 flagellin expression
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Negative DNA supercoiling is required for Hin recombinase-catalyzed inversion
Fis (DNA-bending protein) together with HUenhances inversion ≈ 1000-fold
≈ 60 bp
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Resolving multimeric circular DNA structures by site-specific recombination
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Action of Xer recombinases
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Recombination
• Homologous recombination
• Site-specific recombination
• Transposition
Different sets of proteins are responsible for thedifferent types of DNA exchange.
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Transposition
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Transposons are present in all forms of life
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Table 9.3 Genomes 3 (© Garland Science 2007)
45.8%
Transposons in the human genome
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DNA and RNA transposons