MSH5, A NOVEL MUTS HOMOLOG, FACILITATES MEIOTIC RECIPROCAL RECOMBINATION BETWEEN HOMOLOGS IN SACCHAROMYCES-CEREVISIAE BUT NOT MISMATCH REPAIR

Using a screen designed to identify yeast mutants specifically defecti ve in recombination between homologous chromosomes during meiosis, we have obtained new alleles of the meiosis specific genes, HOP1, RED1, a nd MEK1. In addition, the screen identified a novel gene designated MS H5 (MutS homolog 5). Although Msh5p exhibits strong homology to the Mu tS family of proteins, it is not involved in DNA mismatch repair. Dipl oids lacking the MSH5 gene display decreased levels of spore viability , increased levels of meiosis I chromosome nondisjunction, and decreas ed levels of reciprocal exchange between, but not within, homologs. Ge ne conversion is not reduced. Msh5 mutants are phenotypically similar to mutants in the meiosis-specific gene MSH4 (Ross-Macdonald and Roede r 1994). Double mutant analysis using msh4 msh5 diploids demonstrates that the two genes are in the same epistasis group and therefore are l ikely to function in a similar process-namely, the facilitation of int erhomolog crossovers during meiosis.