INTERACTION BETWEEN MISMATCH REPAIR AND GENETIC-RECOMBINATION IN SACCHAROMYCES-CEREVISIAE

The yeast Saccharomyces cerevisiae encodes a set of genes that show st rong amino acid sequence similarity to MutS and MutL, proteins require d for mismatch repair in Escherichia coli. We examined the role of MSH 2 and PMS1, yeast homologs of mutS and mutL, respectively, in the repa ir of base pair mismatches formed during meiotic recombination. By usi ng specifically marked HIS4 and ARG4 alleles, we showed that msh2 muta nts displayed a severe defect in the repair of all base pair mismatche s as well as 1-, 2- and 4-bp insertion/deletion mispairs. The msh2 and pms1 phenotypes were indistinguishable, suggesting that the wild-type gene products act in the same repair pathway. A comparison of gene co nversion events in wild-type and msh2 mutants indicated that mismatch repair plays an important role in genetic recombination. (1) Tetrad an alysis at five different loci revealed that; in msh2 mutants, the majo rity of aberrant segregants displayed a sectored phenotype, consistent with a failure to repair mismatches created during heteroduplex forma tion. In wild type, base pair mismatches were almost exclusively repai red toward conversion rather than restoration. (2) In msh2 strains 10- 19% of the aberrant tetrads were Ab4:4. (3) Polarity gradients at HIS4 and ARG4 were nearly abolished in msh2 mutants. The frequency of gene conversion at the 3' end of these genes was increased and was nearly the frequency observed at the 5' end. (4) Co-conversion studies were c onsistent with mismatch repair acting to regulate heteroduplex DNA tra ct length. We favor a model proposing that recombination events occur through the formation and resolution of heteroduplex intermediates and that mismatch repair proteins specifically interact with recombinatio n enzymes to regulate the length of symmetric heteroduplex DNA.