The Dmc1 recombinase physically interacts with and promotes the meiotic crossover functions of the Mlh1-Mlh3 endonuclease.

The accurate segregation of homologous chromosomes during the Meiosis I reductional division in most sexually reproducing eukaryotes requires crossing over between homologs. In baker's yeast approximately 80 percent of meiotic crossovers result from Mlh1-Mlh3 and Exo1 acting to resolve double-Holliday junction (dHJ) intermediates in a biased manner. Little is known about how Mlh1-Mlh3 is recruited to recombination intermediates and whether it interacts with other meiotic factors prior to its role in crossover resolution. We performed a haploinsufficiency screen in baker's yeast to identify novel genetic interactors with Mlh1-Mlh3 using sensitized alleles that disrupt the stability of the Mlh1-Mlh3 complex and confer defects in mismatch repair but do not disrupt meiotic crossing over. We identified several genetic interactions between and the recombinase responsible for recombination between homologous chromosomes during meiosis. We then showed that Mlh3 physically interacts with Dmc1 and at times in meiotic prophase when Dmc1 acts as a recombinase. Interestingly, restricting expression to roughly the time of crossover resolution resulted in a null-like phenotype for crossing over. Our data are consistent with a model in which Dmc1 nucleates a polymer of Mlh1-Mlh3 to promote crossing over.