A DMC1 Mutant That Bypasses Accessory Factor Mei5-Sae3 Provides Insight into the Regulation of Filament Length in vivo

During meiosis, homologous recombination repairs programmed DNA double-stranded breaks. Meiotic recombination physically links the homologous chromosomes ("homologs"), creating the tension between them that is required for their segregation. The central recombinase in this process is Dmc1. Dmc1's activity is regulated by its accessory factors including the heterodimeric protein Mei5-Sae3 and Rad51. We use a gain-of-function dmc1 mutant, dmc1-E157D, that bypasses Mei5-Sae3 to gain insight into the role of this accessory factor and its relationship to mitotic recombinase Rad51, which also functions as a Dmc1 accessory protein during meiosis. We find that Mei5-Sae3 has a role in filament formation and stability, but not in the bias of recombination partner choice that favors homolog over sister chromatids. Analysis of meiotic recombination intermediates suggests that Mei5-Sae3 and Rad51 function independently in promoting filament stability. In spite of its ability to load onto single-stranded DNA and carry out recombination in the absence of Mei5-Sae3, recombination promoted by the Dmc1 mutant is abnormal in that it forms foci in the absence of DNA breaks, displays unusually high levels of multi-chromatid and intersister joint molecule intermediates, as well as high levels of ectopic recombination products. Using super-resolution microscopy, we show that the mutant protein forms longer foci than those formed by wild-type Dmc1. To determine whether the in vivo phenotypes were caused by an increased tendency of the Dmc1 mutant to form multi-invasions, an aberrant recombination intermediate in which a single broken molecule engages two more dsDNA donors, we purified Dmc1-E157D and analyzed its strand exchange activity in vitro. Our preliminary results indicate that in comparison to wild-type Dmc1, Dmc1-E157D is hyper-recombinant and forms more multi-invasions in D-loop assays. Our data support a model in which longer filaments are more prone to engage in aberrant recombination events, suggesting that filament lengths are normally limited by a regulatory mechanism that functions to prevent recombination-mediated genome rearrangements.