The Rad51 nucleoprotein filament mediates DNA strand exchange, a key step of homologous recombination. This activity is stimulated by replication protein A (RPA), but only when RPA is introduced after Rad51 nucleoprotein filament formation. In contrast, RPA inhibits Rad51 nucleoprotein complex formation by prior binding to single-stranded DNA (ssDNA), but Rad52 protein alleviates this inhibition. Here we show that Rad51 filament formation is simultaneous with displacement of RPA from ssDNA. This displacement is initiated by a rate-limiting nucleation of Rad51 protein onto ssDNA complex, followed by rapid elongation of the filament. Rad52 protein accelerates RPA displacement by Rad51 protein. This acceleration probably involves direct interactions with both Rad51 protein and RPA. Detection of a Rad52-RPA-ssDNA co-complex suggests that this co-complex is an intermediate in the displacement process.
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