Investigating the roles of the Srs2 and Pif1 helicases in DNA double-strand break repair in Saccharomyces cerevisiae
MetadataShow full item record
DNA double strand breaks (DSBs), which may occur during DNA replication or due to the action of genotoxic agents, are extremely dangerous DNA lesions as they can cause chromosomal rearrangements and cell death. Therefore, accurate DSB repair is vital for genome stability and cell survival. Two main mechanisms serve to repair DNA DSBs: non-homologous end joining, which re-ligates DNA ends together, and homologous recombination (HR), which restores broken DNA using homologous sequence as a template for repair. One-ended DSBs are a subject for the specialised HR-dependent repair pathway known as break-induced replication (BIR). At low frequency, DNA breaks can also be healed by telomerase, which normally extends telomeres at natural chromosome ends, but may also add de novo telomeres to DSBs due to their similarity to chromosome ends. De novo telomere addition is a deleterious event, which is effectively inhibited by the nuclear Pif1 (nPif1) helicase phosphorylated at the TLSSAES motif in response to DNA damage. In this study, it is reported that the same regulatory motif of nPif1 is also required for DSB repair via BIR. The requirement of the nPif1 TLSSAES sequence in BIR is dependent on the functional DNA damage response (DDR). Thus, nPif1 phosphorylation by the DDR machinery might mediate the role of nPif1 in BIR. In contrast, the nPif1 regulatory motif is not essential for BIR at telomeres in cells lacking telomerase. These observations indicate that the mechanism of nPif1 function in DSB repair via BIR and in BIR at telomeres might be different. In this work, a protocol for nPif1 pull-down was optimized to reveal the mechanism of the phosphorylation-dependent nPif1 functions in cells undergoing DNA repair, i. e. the mechanism of nPif1-mediated inhibition of de novo telomere addition and promoting DSB repair via BIR. In future, this protocol can be used to dissect the role of nPif1 in DNA repair through the identification of its potential interacting partners. The Srs2 helicase negatively regulates HR via dismantling Rad51 filaments. According to preliminary data from the laboratory of Sveta Makovets, Srs2 also promotes de novo telomere addition at DSBs in a Rad51-dependent manner. The work presented here establishes that Srs2 is dispensable for telomerase-mediated addition of TG1-3 repeats to DSBs. Instead, Srs2 is required for the reconstitution of the complementary DNA strand after telomerase action, thus ensuring the completion of de novo telomere addition. Overall, this study demonstrates that recombination-dependent DSB repair and de novo telomere addition share common regulatory components, i. e. the nPif1 helicase phosphorylated in response to DNA damage and the Srs2 helicase. Phosphorylated nPif1 promotes DSB repair via BIR in addition to its known role in inhibition of telomerase at DSBs, whereas Srs2 uses its well established ability to remove Rad51 from ssDNA to promote the restoration of dsDNA and thus to complete de novo telomere addition.