Rad55-Rad57 and Srs2 regulate homology search onset, coordination, reach and inactivation.

DNA double-strand break (DSB) repair by homologous recombination entails the coordinated search for a homologous dsDNA molecule by two heterotypic Rad51-ssDNA filaments in eukaryotes. How homology search is regulated in cells remains largely unknown. Using genomic and molecular assays to track spatial chromatin organization and early recombination intermediates, we investigated the roles in homology search of two antagonistic regulators of Rad51-ssDNA filaments metabolism in S. cerevisiae: the Rad51 paralogs Rad55-Rad57 and the 3'-5' ssDNA translocase Srs2. Srs2 promoted the coordinated search between filaments on each DSB ends and inactivated homology search following homology identification. Rad55-Rad57 both stimulated the formation of Rad51-ssDNA filaments and protected them against disruption by Srs2. Together, Rad55-Rad57 and Srs2 enacted a structural proof-reading that resulted in stiffer Rad51-ssDNA filaments competent for genome-wide homology search. This work reveals multiple ways by which the control of Rad51-ssDNA filament metastability by the individual and joint activities of Rad55-Rad57 and Srs2 regulate homology search onset, coordination, reach and inactivation.