Roles for two RecA homologs in promoting meiotic chromosome synapsis.

Previous studies have shown that the RAD51 and DMC1 genes of Saccharomyces cerevisiae encode homologs of the Escherichia coli RecA strand exchange enzyme. Results presented here demonstrate that the dmc1 and rad51 mutants undergo nearly complete chromosome synapsis, but synaptonemal complex formation is delayed substantially compared with wild type. In the zip1 mutant, chromosomes are paired homologously, but not synapsed, and the protein backbones (axial elements) of each pair of chromosomes are connected intimately to each other at a few sites referred to herein as axial associations. dmc1 zip1 and rad51 zip1 double mutants assemble axial elements that are not obviously associated, demonstrating that the Dmc1 and Rad51 proteins are required to establish or stabilize axial associations. We propose that axial associations serve to promote meiotic chromosome synapsis and that the absence of these associations accounts for the delayed and inefficient synapsis observed in dmc1 and rad51 strains. During meiosis in haploid yeast, chromosome synapsis takes place between nonhomologous chromosome segments. In a zip1 haploid, axial associations are not apparent, suggesting that these associations depend on interactions between homologous sequences.