Rad51 paralogs Rad55-Rad57 balance the anti-recombinase Srs2 in Rad51 filament formation

Homologous recombination is a high-fidelity DNA repair pathway. Besides a critical role in accurate chromosome segregation during meiosis, recombination functions in DNA repair and in the recovery of stalled or broken replication forks to ensure genomic stability. In contrast, inappropriate recombination contributes to genomic instability, leading to loss of heterozygosity, chromosome rearrangements, and cell death. The RecA/UvsX/RadA/Rad51 family of proteins catalyzes the signature reactions of recombination, homology search and DNA strand invasion (1,2). Eukaryotes also possess Rad51 paralogs, whose exact role in recombination remains to be defined (3). Here we show that the budding yeast Rad51 paralogs, the Rad55-Rad57 heterodimer, counteract the anti-recombination activity of the Srs2 helicase. Rad55-Rad57 associate with the Rad51-ssDNA filament, rendering it more stable than a nucleoprotein filament containing Rad51 alone. The Rad51/Rad55-Rad57 co-filament resists disruption by the Srs2 anti-recombinase by blocking Srs2 translocation involving a direct protein interaction between Rad55-Rad57 and Srs2. Our results demonstrate an unexpected role of the Rad51 paralogs in stabilizing the Rad51 filament against a biologically important antagonist, the Srs2 anti-recombination helicase. The biological significance of this mechanism is indicated by a complete suppression of the ionizing radiation sensitivity of rad55 or rad57 mutants by concomitant deletion of SRS2, as expected for biological antagonists. We propose that the Rad51 presynaptic filament is a meta-stable reversible intermediate, whose assembly and disassembly is governed by the balance between Rad55-Rad57 and Srs2, providing a key regulatory mechanism controlling the initiation of homologous recombination. These data provide a paradigm for the potential function of the human RAD51 paralogs, which are known to be involved in cancer predisposition and human disease.