Rad51 paralog complex Rad55–Rad57 acts as a molecular chaperone during homologous recombination. Roy et al.
We investigated the role of Rad51 paralog complex Rad55–Rad57 in homologous recombination. We visualized Rad55–Rad57 dynamics at the single–molecule level, in real–time using total internal reflection fluorescence microscopy (TIRFM). Our data shows Rad55–Rad57 binds Rad51–ssDNA only transiently during early stages of Rad51 filament assembly, and strongly stimulates the rate of Rad51–ssDNA filament formation. Release of the paralog complex from the Rad51–ssDNA filament is dependent upon ATP hydrolysis by Rad55–Rad57. We also show that Rad55–Rad57 does not physically block translocation of the anti-recombinase Srs2, but rather counteracts it by promoting rapid Rad51 re-assembly.