Meiotic Crossover Control by Concerted Action of Rad51-Dmc1 in Homolog Template Bias and Robust Homeostatic Regulation

Abstract

During meiosis, repair of programmed DNA double-strand breaks (DSBs) by recombination promotes pairing of homologous chromosomes and their connection by crossovers. Two DNA strand-exchange proteins, Rad51 and Dmc1, are required for meiotic recombination in many organisms. Studies in budding yeast imply that Rad51 acts to regulate Dmc1's strand exchange activity, while its own exchange activity is inhibited. However, in a dmc1 mutant, elimination of inhibitory factor, Hed1, activates Rad51's strand exchange activity and results in high levels of recombination without participation of Dmc1. Here we show that Rad51-mediated meiotic recombination is not subject to regulatory processes associated with high-fidelity chromosome segregation. These include homolog bias, a process that directs strand exchange between homologs rather than sister chromatids. Furthermore, activation of Rad51 does not effectively substitute for Dmc1's chromosome pairing activity, nor does it ensure formation of the obligate crossovers required for accurate homolog segregation. We further show that Dmc1's dominance in promoting strand exchange between homologs involves repression of Rad51's strand-exchange activity. This function of Dmc1 is independent of Hed1, but requires the meiotic kinase, Mek1. Hed1 makes a relatively minor contribution to homolog bias, but nonetheless this is important for normal morphogenesis of synaptonemal complexes and efficient crossing-over especially when DSB numbers are decreased. Super-resolution microscopy shows that Dmc1 also acts to organize discrete complexes of a Mek1 partner protein, Red1, into clusters along lateral elements of synaptonemal complexes; this activity may also contribute to homolog bias. Finally, we show that when interhomolog bias is defective, recombination is buffered by two feedback processes, one that increases the fraction of events that yields crossovers, and a second that we propose involves additional DSB formation in response to defective homolog interactions. Thus, robust crossover homeostasis is conferred by integrated regulation at initiation, strand-exchange and maturation steps of meiotic recombination.

Local chromosome context is a major determinant of crossover pathway biochemistry during budding yeast meiosis

“…VDE-initiated recombinants formed at high frequencies at both HIS4 and URA3, and NCOs exceeded COs by approximately twofold at HIS4 and threefold at URA3 ( Figure 2C). These values are within the range observed in genetic studies of Spo11-induced gene conversion in budding yeast (Fogel et al, 1979), but differ from the average of near-parity between NCOs and COs observed in molecular assays (Lao et al, 2013;Martini et al, 2006). This is Figure 2C from HIS4 insert-containing mutants lacking MutLg (mlh3), structure-selective nucleases (mms4-md yen1 slx1) or both resolvase activities (mlh3 mms4-md yen1 slx1).…”

DNA damage response clamp 9-1-1 promotes assembly of ZMM proteins for formation of crossovers and synaptonemal complex

“…Therefore, although the checkpoint clamp and Rad51 were recruited to single-stranded DSB ends independently of one another (Shinohara et al, 2003a), the two components function concurrently at the same sites to promote the progression of recombination intermediates during meiosis. In the absence of Rad51, intersister joint molecules were increased and interhomolog joint molecules decreased to a similar extent, suggesting a simple redistribution of joint molecule intermediates from homolog to sister (Hong et al, 2013;Lao et al, 2013;Schwacha and Kleckner, 1997). Taken together, these findings delineate a crucial role for Rad51-Dmc1 cooperation in interhomolog bias for meiotic recombination.…”

“…However, this mechanism is unlikely because, unlike clamp-defective mutants, both rad51 and dmc1 mutants are proficient in Zip3-focus formation ( Fig. 5; Lao et al, 2013). These findings led us to the hypothesis that the 9-1-1 clamp, in addition to its role in homolog bias, is necessary for the efficient recruitment and/or stabilization of ZMM proteins along the developing synaptonemal complex.…”

Functional Relationship of ATP Hydrolysis, Presynaptic Filament Stability, and Homologous DNA Pairing Activity of the Human Meiotic Recombinase DMC1

“…Recent work has provided evidence that the essential role of Rad51 in meiosis is to enhance the recombinase activity of Dmc1 (10). These results thus reveal distinct functions of Rad51 and Dmc1 in meiotic HR (11)(12)(13). In mice, a homozygous deletion of Rad51 causes embryonic lethality.…”