Controlling Meiotic Recombinational Repair – Specifying the Roles of ZMMs, Sgs1 and Mus81/Mms4 in Crossover Formation

Abstract

Crossovers (COs) play a critical role in ensuring proper alignment and segregation of homologous chromosomes during meiosis. How the cell balances recombination between CO vs. noncrossover (NCO) outcomes is not completely understood. Further lacking is what constrains the extent of DNA repair such that multiple events do not arise from a single double-strand break (DSB). Here, by interpreting signatures that result from recombination genome-wide, we find that synaptonemal complex proteins promote crossing over in distinct ways. Our results suggest that Zip3 (RNF212) promotes biased cutting of the double Holliday-junction (dHJ) intermediate whereas surprisingly Msh4 does not. Moreover, detailed examination of conversion tracts in sgs1 and mms4-md mutants reveal distinct aberrant recombination events involving multiple chromatid invasions. In sgs1 mutants, these multiple invasions are generally multichromatid involving 3–4 chromatids; in mms4-md mutants the multiple invasions preferentially resolve into one or two chromatids. Our analysis suggests that Mus81/Mms4 (Eme1), rather than just being a minor resolvase for COs is crucial for both COs and NCOs in preventing chromosome entanglements by removing 3′- flaps to promote second-end capture. Together our results force a reevaluation of how key recombination enzymes collaborate to specify the outcome of meiotic DNA repair.

Meiotic Recombination: The Essence of Heredity

“…Genetic studies provide compelling evidence that MUS81 enzymes can process meiotic joint molecules in vivo, although the exact substrate(s) remains uncertain (Boddy et al 2001;Osman et al 2003;Cromie et al 2006;Berchowitz et al 2007;Higgins et al 2008a;Holloway et al 2008;Jessop and Lichten 2008;Oh et al 2008;Hickson and Mankouri 2011;Oke et al 2014). The cleavage patterns observed with joint molecule substrates in vitro and the predominant role of Mus81 -Eme1 in promoting crossovers in fission yeast has led to a sequential D-loop nicking model that specifically produces crossovers (Osman et al 2003;Gaskell et al 2007).…”

“…Analysis in budding yeast reveals an early function for Mus81 -Mms4 in the efficient formation of interhomolog joint molecules, in addition to a late function in joint-molecule resolution (De Los Santos et al 2003;Oh et al 2008;Matos et al 2011). A general, early function for Mus81 -Mms4 is further supported by the increased frequency of gene conversions and longer gene-conversion tracts seen in mus81/ mms4 mutants (De Los Santos et al 2003;Oke et al 2014). These phenotypes are suggested to derive from aberrant processing of 3 0 -flaps formed when extended DSB ends anneal (Fig.…”

“…In the absence of STR, intersister joint molecules and aberrant "multi-chromatid" structures comprising three and four interconnected chromosomes become prominent intermediates (Oh et al 2007). Consequently, joint-molecule resolution and formation of both crossovers and noncrossovers becomes deregulated, acutely dependent on Ndt80 and the structure-selective endonucleases (Mus81 -Mms4, Slx1 -Slx4 and Yen1, discussed further below), and independent of MutSg and MutLg, which define the class I crossover pathway (Jessop et al 2006;Oh et al 2007Oh et al , 2008Jessop and Lichten 2008;De Muyt et al 2012;Zakharyevich et al 2012;Oke et al 2014;Kaur et al 2015;Tang et al 2015). Thus, STR plays both a procrossover role, specifically facilitating formation of class I crossovers, and promotes noncrossover formation via synthesis-dependent strand annealing (Fig.…”

Roles for mismatch repair family proteins in promoting meiotic crossing over

“…In vitro , Mus81-Mms4 nicks flapped DNA substrates and four-way Holliday junctions, among other branch structures [122,123]. This complex has been implicated as the primary resolvase in a crossover pathway that does not require ZMM proteins to stabilize crossover intermediates (Figure 3C), and has been suggested to prevent chromosome entanglements in both crossover and non-crossover pathways [102,115,124]. In both in vitro and in in vivo assays, Mus81-Mms4 is activated by phosphorylation by Cdc5 kinase, suggesting that its meiotic resolvase activity is carefully regulated through post-translational modification [125,126].…”

“…A recent bioinformatic study further suggests that an MSH/MLH complex may not be required for imposing asymmetric nicking of dHJs [124]. Sequencing of recombination products in yeast tetrads derived from msh4 strains showed that biased nicking of dHJs was retained.…”