The Budding Yeast Mei5 and Sae3 Proteins Act Together With Dmc1 During Meiotic Recombination

Here we provide evidence that the Mei5 and Sae3 proteins of budding yeast act together with Dmc1, a meiosis-specific, RecA-like recombinase. The mei5 and sae3 mutations reduce sporulation, spore viability, and crossing over to the same extent as dmc1. In all three mutants, these defects are largely suppressed by overproduction of Rad51. In addition, mei5 and sae3, like dmc1, suppress the cell-cycle arrest phenotype of the hop2 mutant. The Mei5, Sae3, and Dmc1 proteins colocalize to foci on meiotic chromosomes, and their localization is mutually dependent. The localization of Rad51 to chromosomes is not affected in either mei5 or sae3. Taken together, these observations suggest that the Mei5 and Sae3 proteins are accessory factors specific to Dmc1. We speculate that Mei5 and Sae3 are necessary for efficient formation of Dmc1-containing nucleoprotein filaments in vivo

The Mnd1 Protein Forms a Complex with Hop2 To Promote Homologous Chromosome Pairing and Meiotic Double-Strand Break Repair

The hop2 mutant of Saccharomyces cerevisiae arrests in meiosis with extensive synaptonemal complex (SC) formation between nonhomologous chromosomes. A screen for multicopy suppressors of a hop2-ts allele identified the MND1 gene. The mnd1-null mutant arrests in meiotic prophase, with most double-strand breaks (DSBs) unrepaired. A low level of mature recombinants is produced, and the Rad51 protein accumulates at numerous foci along chromosomes. SC formation is incomplete, and homolog pairing is severely reduced. The Mnd1 protein localizes to chromatin throughout meiotic prophase, and this localization requires Hop2. Unlike recombination enzymes such as Rad51, Mnd1 localizes to chromosomes even in mutants that fail to initiate meiotic recombination. The Hop2 and Mnd1 proteins coimmunoprecipitate from meiotic cell extracts. These results suggest that Hop2 and Mnd1 work as a complex to promote meiotic chromosome pairing and DSB repair. The identification of Hop2 and Mnd1 homologs in other organisms suggests that the function of this complex is conserved among eukaryotes

A Novel Nonnull ZIP1 Allele Triggers Meiotic Arrest With Synapsed Chromosomes in Saccharomyces cerevisiae

During meiotic prophase, assembly of the synaptonemal complex (SC) brings homologous chromosomes into close apposition along their lengths. The Zip1 protein is a major building block of the SC in Saccharomyces cerevisiae. In the absence of Zip1, SC fails to form, cells arrest or delay in meiotic prophase (depending on strain background), and crossing over is reduced. We created a novel allele of ZIP1, zip1-4LA, in which four leucine residues in the central coiled-coil domain have been replaced by alanines. In the zip1-4LA mutant, apparently normal SC assembles with wild-type kinetics; however, crossing over is delayed and decreased compared to wild type. The zip1-4LA mutant undergoes strong checkpoint-induced arrest in meiotic prophase; the defect in cell cycle progression is even more severe than that of the zip1 null mutant. When the zip1-4LA mutation is combined with the pch2 checkpoint mutation, cells sporulate with wild-type efficiency and crossing over occurs at wild-type levels. This result suggests that the zip1-4LA defect in recombination is an indirect consequence of cell cycle arrest. Previous studies have suggested that the Pch2 protein acts in a checkpoint pathway that monitors chromosome synapsis. We hypothesize that the zip1-4LA mutant assembles aberrant SC that triggers the synapsis checkpoint