The Function of Exonuclease I in Meiotic Recombination: A Genetic and Physical Analysis

Exo1 is a member of the Rad2 protein family and possesses both 5’-3’ exonuclease and 5’ flap endonuclease activities. In addition to performing a variety of functions during mitotic growth, Exo1 is also important for the production of crossovers during meiosis. However, its precise molecular role has remained ambiguous and several models have been proposed to account for the crossover deficit observed in its absence. Here, physical evidence that the nuclease activity of Exo1 is essential for normal 5’-3’ resection at the Spo11-dependent HIS4 hotspot in otherwise wild-type cells is presented. This same activity was also required for normal levels of gene conversion at the locus. Nevertheless, gene conversions were frequently observed at a distance beyond that at which resection was readily detectable arguing that it is not the extent of the initial DNA end resection that limits heteroduplex formation. In addition to these nuclease-dependent functions, nuclease-deficient exo1 mutants were found to be capable of maintaining crossing-over at wild-type levels in a number of genetic intervals, suggesting that Exo1 also plays a nuclease-independent role in crossover promotion. Furthermore, the results of both physical and genetic analyses imply that Sgs1 does not contribute significantly to resection during meiosis in exo1∆ cells, indicating that the mitotic and meiotic resection machinery differs. In light of these new insights, a model describing the formation of heteroduplex DNA and crossovers during meiosis is proposed.