Using a screen designed to identify yeast mutants specifically defective in recombination between homologous chromosomes during meiosis, we have obtained new alleles of the meiosis-specific genes, HOP1, RED1, and MEK1. In addition, the screen identified a novel gene designated MSH5 (MutS homolog 51. Although Msh5p exhibits strong homology to the MutS family of proteins, it is not involved in DNA mismatch repair. Diploids lacking the MSH5 gene display decreased levels of spore viability, increased levels of meiosis I chromosome nondisjunction, and decreased levels of reciprocal exchange between, but not within, homologs. Gene conversion is not reduced. Msh5 mutants are phenotypicaUy similar to mutants in the meiosis-specific gene MSH4 (Ross-Macdonald and Roeder 1994}. Double mutant analysis using rash4 rash5 diploids demonstrates that the two genes are in the same epistasis group and therefore are likely to function in a similar process--namely, the facilitation of interhomolog crossovers during meiosis.
During meiosis, axial elements are generated by the condensation of sister chromatids along a protein core as precursors to the formation of the synaptonemal complex (X). Functional axial elements are essential for wild-type levels of recombination and proper reductional segregation at meiosis I. Genetic and cytological data suggest that three meiosis-specific genes, HOP1, RED1 and MEK1, are involved in axial element formation in the yeast Saccharomyces cerevisiae. HOP1 and RED1 encode structural components of axial elements while MEK1 encodes a putative protein kinase. Using a partially functional allele of MEK1, new genetic interactions have been found between HOP1, RED1 and MEK1. Overexpression of HOP1 partially suppresses the spore inviability and recombination defects of mekl-974; in contrast, overexpression of RED1 exacerbates the mek1-974 spore inviability. Co-overexpression of HOP1 and RED1 in mek1-974 diploids alleviates the negative effect of overexpressing RED1 alone. Red1p/Red1p as well as Hop1p/Red1p interactions have been reconstituted in two hybrid experiments. Our results suggest a model whereby Mekl kinase activity controls axial element assembly by regulating the affinity with which Hoplp and Redlp interact with each other.
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