Sir3 Heterochromatin Protein Promotes NHEJ by Direct Inhibition of Sae2

Heterochromatin is a conserved feature of eukaryotic chromosomes, with central roles in gene expression regulation and maintenance of genome stability. How heterochromatin proteins regulate DNA repair remains poorly described. In Saccharomyces cerevisiae , the Silent Information Regulator (SIR) complex assembles heterochromatin-like chromatin at subtelomeres. SIR-mediated repressive chromatin limits double strand break (DSB) resection protecting damaged chromosome ends during HR. As resection initiation marks the cross-road between repair by non-homologous end joining (NHEJ) or HR, we asked whether SIR- mediated heterochromatin regulates NHEJ. We show that SIRs promote NHEJ through two pathways, one depending on repressive chromatin assembly, and the other relying on Sir3 in a manner that is independent of its heterochromatin-promoting function. Sir3 physically interacts with Sae2 and this interaction impairs Sae2-dependent MRX functions. As a consequence, Sir3 limits Mre11-mediated resection, delays MRX removal from DSB ends and promotes NHEJ. ### Competing Interest Statement The authors have declared no competing interest.