Exo1-protected DNA nicks direct crossover formation in meiosis

In most sexually reproducing organisms crossing over between chromosome homologs during meiosis is critical for the viability of haploid gametes. Most crossovers that form in meiosis in budding yeast result from the biased resolution of double Holliday Junction (dHJ) intermediates. This dHJ resolution step involves the actions Rad2/XPG family nuclease Exo1 and the Mlh1-Mlh3 mismatch repair endonuclease. At present little is known about how these factors act in meiosis at the molecular level. Here we show that Exo1 promotes meiotic crossing over by protecting DNA nicks from ligation. We found that structural elements in Exo1 required for interactions with DNA, such as bending of DNA during nick/flap recognition, are critical for its role in crossing over. Consistent with these observations, meiotic expression of the Rad2/XPG family member Rad27 partially rescued the crossover defect in exo1 null mutants, and meiotic overexpression of Cdc9 ligase specifically reduced the crossover levels of exo1 DNA binding mutants to levels approaching the exo1 null. In addition, our work identified a role for Exo1 in crossover interference that appears independent of its resection activity. Together, these studies provide experimental evidence for Exo1-protected nicks being critical for the formation of meiotic crossovers and their distribution.