GCN5 and TADA2B constitutively regulate XRCC1 function during DNA repair to maintain cell survival.

XRCC1 orchestrates base excision repair (BER) and single-strand break repair (SSBR) through protein-protein interactions, with disruption contributing to neurological diseases, including spinocerebellar ataxia autosomal recessive 26 (SCAR26). While PARP1-mediated recruitment is well established, constitutive regulatory mechanisms have remained unclear. Here, we identify GCN5 and TADA2B, components of the SAGA histone acetyltransferase complex, as novel XRCC1 binding partners providing constitutive regulation of repair localization. These proteins bind XRCC1 via distinct BRCT domain interactions-GCN5 to BRCT I, TADA2B to BRCT II-independent of DNA damage or GCN5 acetyltransferase activity. Depletion of either protein impairs DNA repair efficiency, sensitizes cells to genotoxic stress, and increases cell death, demonstrating their integral role in cell survival. Unexpectedly, GCN5 or TADA2B deficiency rescues focal retention defects in BRCT II deletion mutants, revealing how constitutive interactions optimize normal XRCC1 function but become counterproductive when XRCC1 is structurally compromised. In addition, the SCAR26-associated BRCT II point mutation disrupts TADA2B binding, yet paradoxically, this normally beneficial interaction becomes inhibitory in the mutant context. Our findings establish a ready-for-action model where repair complexes preorganize machinery before DNA damage occurs, revealing how constitutive regulation determines cellular responses to DNA damage.