Rad54 protein is targeted to pairing loci by the Rad51 nucleoprotein filament

Rad51 and Rad54 proteins are important for the repair of double-stranded DN A (dsDNA) breaks by homologous recombination in eukaryotes. Rad51 assembles on single-stranded DNA (ssDNA) to form a helical nucleoprotein filament th at performs homologous pairing with dsDNA; Rad54 stimulates this pairing su bstantially. Here, we demonstrate that Rad54 acts in concert with the matur e Rad51-ssDNA filament. Enhancement of DNA pairing by Rad54 is greatest at an equimolar ratio relative to Rad51 within the filament. Reciprocally, the Rad51-ssDNA filament enhances both the dsDNA-dependent ATPase and the dsDN A unwinding activities of Rad54. We conclude that Rad54 participates in the DNA homology search as a component of the Rad51-nucleoprotein filament and that the filament delivers Rad54 to the dsDNA pairing locus, thereby linki ng the unwinding of potential target DNA with the homology search process.