Abstract
Genetic recombination in all kingdoms of life initiates when helicases and nucleases process (resect) thefree DNA ends to expose single-stranded DNA (ssDNA) overhangs. Resection regulation in bacteria is programmed by a DNA sequence, but a general mechanism limiting resection in eukaryotes has remained elusive. Using single-molecule imaging of reconstituted human DNA repair factors, we identifyphosphorylated RPA (pRPA) as a negative resection regulator. Bloom's syndrome (BLM) helicase together with exonuclease 1 (EXO1) and DNA2 nucleases catalyze kilobase-length DNA resection on nucleosome-coated DNA. The resulting ssDNA is rapidly bound by RPA, which further stimulates DNA resection. RPA is phosphorylated during resection as part of the DNA damage response (DDR). Remarkably, pRPA inhibits DNA resection in cellular assays and invitro via inhibition of BLM helicase. pRPA suppresses BLM initiation at DNA ends and promotes the intrinsic helicase strand-switching activity. These findings establish that pRPA providesa feedback loop between DNA resection and the DDR.
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