Originally published as Genetics Published Articles Ahead of Print on May 15, 2006.

Genetics, Vol. 173, 1223-1239, July 2006, Copyright © 2006
doi:10.1534/genetics.106.055616

Analysis of the Proteins Involved in the in Vivo Repair of Base–Base Mismatches and Four-Base Loops Formed During Meiotic Recombination in the Yeast Saccharomyces cerevisiae

* Department of Molecular Genetics and Microbiology, Duke University Medical Center, Durham, North Carolina 27710 and {dagger} Curriculum in Genetics and Molecular Biology and Department of Biology, University of North Carolina, Chapel Hill, North Carolina 27599-3280

1 Corresponding author: Department of Molecular Genetics and Microbiology, Duke University Medical Center, Durham, NC 27710-3054.
E-mail: tom.petes{at}duke.edu

DNA mismatches are generated when heteroduplexes formed during recombination involve DNA strands that are not completely complementary. We used tetrad analysis in Saccharomyces cerevisiae to examine the meiotic repair of a base–base mismatch and a four-base loop in a wild-type strain and in strains with mutations in genes implicated in DNA mismatch repair. Efficient repair of the base–base mismatch required Msh2p, Msh6p, Mlh1p, and Pms1p, but not Msh3p, Msh4p, Msh5p, Mlh2p, Mlh3p, Exo1p, Rad1p, Rad27p, or the DNA proofreading exonuclease of DNA polymerase {delta}. Efficient repair of the four-base loop required Msh2p, Msh3p, Mlh1p, and Pms1p, but not Msh4p, Msh5p, Msh6p, Mlh2p, Mlh3p, Exo1p, Rad1p, Rad27p, or the proofreading exonuclease of DNA polymerase {delta}. We find evidence that a novel Mlh1p-independent complex competes with an Mlhp-dependent complex for the repair of a four-base loop; repair of the four-base loop was affected by loss of the Mlh3p, and the repair defect of the mlh1 and pms1 strains was significantly smaller than that observed in the msh2 strain. We also found that the frequency and position of local double-strand DNA breaks affect the ratio of mismatch repair events that lead to gene conversion vs. restoration of Mendelian segregation.




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