Rad51 paralog complex Rad55-Rad57 acts as a molecular chaperone during homologous recombination

Homologous recombination (HR) is essential for the maintenance of genome integrity. Rad51 paralogs fulfill a conserved, but undefined role in HR, and their mutations are associated with increased cancer risk in humans. Here, we use single-molecule imaging to reveal that the Saccharomyces cerevisiae Rad51 paralog complex Rad55-Rad57 promotes the assembly of Rad51 recombinase filaments through transient interactions, providing evidence that it acts as a classical molecular chaperone. Srs2 is an ATP-dependent anti-recombinase that downregulates HR by actively dismantling Rad51 filaments. Contrary to the current model, we find that Rad55- Rad57 does not physically block the movement of Srs2. Instead, Rad55-Rad57 promotes rapid re- assembly of Rad51 filaments after their disruption by Srs2. Our findings support a model in which Rad51 is in flux between free and ssDNA-bound states, the rate of which is dynamically controlled though the opposing actions of Rad55-Rad57 and Srs2.