Recombination is a component of DNA metabolism and is important for the repair of damage such as double-strand breaks (DSBs). However, aberrant recombination can lead to increased mutation rates and must be regulated. BLM, a member of the conserved family of RecQ helicases, is one factor that negatively regulates recombination. In Saccharomyces cerevisiae we found that the second end of a DSB is actively engaged in strand invasion as opposed to passively awaiting annealing to the first end as presented by the canonical model for DSB repair (DSBR) in meiosis. Additionally, in mutants of Sgs1, the budding yeast RecQ ortholog, aberrant multi-chromatid DNA intermediates formed by promiscuous invasion of the second end into multiple templates are stabilized. We present a mechanism in which Sgs1 promotes disassembly of aberrant DNA intermediates, thereby decreasing crossing-over during meiosis. Mutations in either component of the Mus81-Mms4 heterodimer, an XPF-ERCC1-like endonuclease, are synthetically lethal with the sgs1 mutant in a recombination-dependent manner. We also find a subset of unresolved recombination intermediates prevent proper meiotic progression in the sgs1 mms4 double mutant. Instead of a redundancy in function with the RecQ helicase, our evidence suggests Mus81-Mms4 acts to resolve aberrant DNA intermediates that bypass Sgs1 regulation.
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