Delineation of Joint Molecule Resolution Pathways in Meiosis Identifies a Crossover-Specific Resolvase

At the final step of homologous recombination, Holliday junction-containing joint molecules (JMs) are resolved to form crossover or noncrossover products. The enzymes responsible for JM resolution in vivo remain uncertain, but three distinct endonucleases capable of resolving JMs in vitro have been identified: Mus81-Mms4(EME1), Slx1-Slx4(BTBD12), and Yen1(GEN1). Using physical monitoring of recombination during budding yeast meiosis, we show that all three endonucleases are capable of promoting JM resolution in vivo. However, in mms4 slx4 yen1 triple mutants, JM resolution and crossing over occur efficiently. Paradoxically, crossing over in this background is strongly dependent on the Blooms helicase ortholog Sgs1, a component of a well-characterized anticrossover activity. Sgs1-dependent crossing over, but not JM resolution per se, also requires XPG family nuclease Exo1 and the MutLγ complex Mlh1-Mlh3. Thus, Sgs1, Exo1, and MutLγ together define a previously undescribed meiotic JM resolution pathway that produces the majority of crossovers in budding yeast and, by inference, in mammals. [Display omitted] ► Efficient joint molecule resolution occurs without Mus81, Yen1, and Slx1 nucleases ► Helicase Sgs1 has unanticipated roles in both crossover and noncrossover formation ► Exo1-MutLγ is a crossover-specific joint molecule resolution factor ► Mus81, Slx1, Yen1, Sgs1, and Exo1-MutLγ account for all resolution in vivo In vivo analysis of joint molecule resolution in budding yeast uncovers unsuspected roles for the helicase Sgs1 in crossover and noncrossover resolution in meiosis and reveals that the majority of crossovers are resolved via a pathway comprised of Sgs1 and the nucleases Exo1 and MutLγ.