Mutation of a meiosis-specific MutS homolog decreases crossing over but not mismatch correction.

MSH4 is a novel meiosis-specific gene required for wild-type levels of spore viability in S. cerevisiae. The predicted product of the MSH4 gene is homologous to the MutS family of proteins; however, msh4-null mutants have no apparent defect in mismatch repair. msh4 mutant strains display wild-type levels of gene conversion and postmeiotic segregation, but they show a reduction in crossing over and a resultant increase in nondisjunction of homologous chromosomes at meiosis I. Immunofluorescence experiments demonstrate that the Msh4 protein is localized to discrete sites on pachytene chromosomes. We propose that Msh4 interacts with a recombination intermediate to influence its resolution.