| Homologous recombination is the predominant mode of DNA double-strand break (DSB) repair in Saccharomyces cerevisiae. When both ends of a break share homology with another locus in the genome, repair occurs by gene conversion (GC). However, if homology to only one of the DSB ends is present, repair occurs by break-induced replication (BIR). While GC is mostly error-free, repair by BIR is often associated with nonreciprocal translocations. Therefore, channeling of the DSBs to the correct repair pathway is crucial for maintaining genome integrity. In this thesis, I describe the identification and characterization of a "Recombination Execution Checkpoint" (REC) that governs the choice between these two repair pathways by determining the relative position and orientation of the homologous sequences used for repair. This REC-mediated choice between GC and BIR is made prior to the initiation of new DNA synthesis from the synapsed DSB ends, and is regulated by the Sgs1 and Mph1 helicases. Although the REC can determine whether the DSB ends are engaged in a GC-compatible configuration, it cannot determine whether or not the ends that are engaged in such a configuration belong to the same break. I also show that gap repair is fundamentally different from break repair, both kinetically and genetically, as Pol32 is required only for gap repair. Overall, these findings greatly enhance our understanding of the regulation of the DNA repair pathways, which is critical for the maintenance and stable inheritance of the genome. |
