Rad51, Rad54, and ZMM proteins antagonize the mismatch repair system to promote fertility of budding yeast intraspecies hybrid zygotes

Rad51 and meiosis-specific Dmc1 catalyze homologous recombination (HR) between maternal and paternal chromosomes during meiosis in many sexual eukaryotes, generating three interhomolog (IH) recombination products: noncrossovers (NCOs), class I crossovers (COs), and class II COs. Some COs form chiasmata, which physically connect homologous chromosomes and ensure proper chromosome segregation during meiosis I. Meiosis is highly relevant to speciation, with the mismatch repair (MMR) system believed to prevent IH-HR, leading to postzygotic isolation between closely related species. We report that several Saccharomyces cerevisiae HR proteins exhibit anti-MMR activities, including Rad51, Rad54, and synapsis-promoting ZMM proteins (Mer3, Zip1, Zip4, and Msh4) in SK1/S288c hybrid zygotes. Srs2 (an ortholog of Escherichia coli helicase UvrD) facilitates MMR by dissembling Rad51-ssDNA presynaptic filaments. Rad51 antagonizes MMR and Srs2 to catalyze D-loop formation. Rad54’s anti-MMR activity acts after Srs2 and outcompetes its pro-HR function to promote Rad51-mediated IH-HR in hybrid zygotes. Dmc1 does not possess anti-MMR activity, but exhibits better mismatch tolerability than Rad51. Following D-loop formation mediated by Dmc1 and/or Rad51, ZMM proteins promote class I IH-CO formation while limiting MMR to promote NCO formation by Sgs1 (an ortholog of E. coli RecQ helicase) and prevent class II IH-CO formation by the Mms4•Mus81 endonuclease. Highlights MMR and HR antagonize each other in intraspecies hybrid zygotes. The HR proteins Rad51, Rad54, and ZMM exhibit anti-MMR activities. MMR overrides HR in hybrid zygotes with anti-MMR mutations, causing hybrid sterility.