OUP accepted manuscript

Creating access to DNA double-strand break (DSB) sites in the chromatin context is an essential step during the repair process, but much remains to be determined about its regulatory mechanisms. Here, using a novel reporter cassette for simultaneous detection of homologous recombination (HR) and nonhomologous end joining (NHEJ) at the same chromosomal site, we report that the efficiency of HR but not NHEJ negatively correlates with nucleosome density. We demonstrate that PARP1 is required for HR by modulating nucleosome density at damage sites. Mechanistic studies indicate that the ATPase domain of BRG1 and the ZnF domain of SIRT1 interact with poly-ADP ribose (PAR) in response to DNA damage, and are responsible for bringing the two factors to broken DNA ends. At DNA damage sites, BRG1 and SIRT1 physically interact, whereupon SIRT1 deacetylates BRG1 at lysine residues 1029 and 1033, stimulating its ATPase activity to remodel chromatin and promote HR.