Sae2 antagonizes Rad9 accumulation at DNA double-strand breaks to attenuate checkpoint signaling and facilitate end resection

The Mre11-Rad50-Xrs2(NBS1) complex plays important roles in the DNA damage response by activating the Tel1(ATM) kinase and catalyzing 5'-3' resection at DNA double-strand breaks (DSBs). To initiate resection, Mre11 endonuclease nicks the 5' strands at DSB ends in a reaction stimulated by Sae2(CtIP). Accordingly, Mre11-nuclease deficient (mre11-nd) and sae2 Delta mutants are expected to exhibit similar phenotypes; however, we found several notable differences. First, sae2 Delta cells exhibit greater sensitivity to genotoxins than mre11-nd cells. Second, sae2 Delta is synthetic lethal with sgs1 Delta, whereas the mre11-nd sgs1 Delta mutant is viable. Third, Sae2 attenuates the Tel1-Rad53(CHK2) checkpoint and antagonizes Rad9(53BP1) accumulation at DSBs independent of Mre11 nuclease. We show that Sae2 competes with other Tel1 substrates, thus reducing Rad9 binding to chromatin and to Rad53. We suggest that persistent Sae2 binding at DSBs in the mre11-nd mutant counteracts the inhibitory effects of Rad9 and Rad53 on Exo1 and Dna2-Sgs1-mediated resection, accounting for the different phenotypes conferred by mre11-nd and sae2 Delta mutations. Collectively, these data show a resection initiation independent role for Sae2 at DSBs by modulating the DNA damage checkpoint.