Rad51, the lead in mitotic recombinational DNA repair, plays a supporting role in budding yeast meiosis

Proteins of the RecA family carry out the central reaction in homologous recombination by forming stretches of hybrid DNA that connect two identical or closely related DNA duplexes. During the mitotic cell cycle, formation of hybrid DNA can serve to align sequences for accurate repair of DNA double-strand breaks or damaged replication forks. During meiosis, recombination serves to create new combinations of alleles and to create connections between homologous chromosomes as required for reductional chromosome segregation. In all eukaryotic organisms, Rad51, a relative of the E. coli RecA protein, is the sole strand exchange protein responsible for mitotic recombinational repair of DNA. Rad51 is also critical for meiotic recombination, and thus rad51 mutants are sterile. Many eukaryotic organisms, including fungi, plants and vertebrates, encode a second RecA-like strand exchange protein, Dmc1, which is meiosis-specific. A key area of research in the recombination field is to determine the functional relationship between Rad51 and Dmc1 during normal meiosis. Earlier studies used mutant strains to show that Rad51 and Dmc1 each can act alone to promote meiotic recombination, but also provided evidence that they often cooperate during a single recombination event. Biochemical studies showed that both proteins have RecA-like homology search and strand exchange activity. Why then do meiotic recombination events in many organisms involve two strand exchange proteins? New observations in budding yeast make it clear that Dmc1, not Rad51, catalyzes homology search and strand exchange for most, if Rad51, the lead in mitotic recombinational DNA repair, plays a supporting role in budding yeast meiosis