THE SEP1 MUTANT OF SACCHAROMYCES-CEREVISIAE ARRESTS IN PACHYTENE AND IS DEFICIENT IN MEIOTIC RECOMBINATION

Strand exchange protein 1 (Sep1) from Saccharomyces cerevisiae promote s homologous pairing of DNA in vitro and sep1 mutants display pleiotro pic phenotypes in both vegetative and meiotic cells. In this study, we examined in detail the ability of the sep1 mutant to progress through meiosis I prophase and to undergo meiotic recombination. In meiotic r eturn-to-growth experiments, commitment to meiotic recombination began at the same time in wild type and mutant; however, recombinants accum ulated at decreased rates in the mutant. Gene conversion eventually re ached nearly wild-type levels, whereas crossing over reached 15-50% of wild type. In an assay of intrachromosomal pop-out recombination, the sep1, dmc1 and rad51 single mutations had only small effects; however , pop-out recombination was virtually eliminated in the sep1 dmc1 and sep1 rad51 double mutants, providing evidence for multiple recombinati on pathways. Analysis of meiotic recombination intermediates indicates that the sep1 mutant is deficient in meiotic double-strand break repa ir. In a physical assay, the formation of mature reciprocal recombinan ts in the sep1 mutant was delayed relative to wild type and ultimately reached only 50% of the wild-type level. Electron microscopic analysi s of meiotic nuclear spreads indicates that the sep1 Delta mutant arre sts in pachytene, with apparently normal synaptonemal complex. This ar rest is RAD9-independent. We hypothesize that the Sep1 protein partici pates directly in meiotic recombination and that other strand exchange enzymes, acting in parallel recombination pathways, are able to subst itute partially for the absence of the Sep1 protein.