EXO1 and MSH6 are high-copy suppressors of conditional mutations in the MSH2 mismatch repair gene of Saccharomyces cerevisiae

In Saccharomyces cerevisiae, Msh2p, a central component in mismatch repair, forms a heterodimer with Msh3p to repair small insertion/deletion mismatch es and with Msh6p to repair base pair mismatches and single-nucleotide inse rtion/deletion mismatches. In haploids, a msh2 Delta mutation is synthetica lly lethal with pol3-01, a mutation in the Pol delta proofreading exonuclea se. Six conditional alleles of msh2 were identified as those that conferred viability in pol3-01 strains at 26 degrees but not at 35 degrees. DNA sequ encing revealed that mutations in several of the msh2(v) alleles are locate d in regions with previously unidentified functions. The conditional inviab ility of two mutants, msh2-L560S pol3-01 and msh2-L910P pol3-01, was suppre ssed by overexpression of EXO1 and MSH6, respectively. Partial suppression was also observed for the temperature-sensitive mutator phenotype exhibited by msh2-L560S and msh2-L910P strains in the lys2-Bgl reversion assay. High -copy plasmids bearing mutations in the conserved EXO1 nuclease domain were unable to suppress,msh2-L560S pol3-01 conditional lethality. These results , in combination with a genetic analysis of msh6 Delta pol3-01 and msh3 Del ta pol3-01 strains, suggest that the activity of the Msh2p-Msh6p heterodime r is important for viability in the presence of the Pol3-01 mutation and th at Exo1p plays a catalytic role in Msh2p-mediated mismatch repair.