Sgs1 Binding to Rad51 Stimulates Homology-Directed DNA Repair in .

Accurate repair of DNA breaks is essential to maintain genome integrity and cellular fitness. Sgs1, the sole member of the RecQ family of DNA helicases in , is important for both early and late stages of homology-dependent repair. Its large number of physical and genetic interactions with DNA recombination, repair, and replication factors has established Sgs1 as a key player in the maintenance of genome integrity. To determine the significance of Sgs1 binding to the strand-exchange factor Rad51, we have identified a single amino acid change at the C-terminal of the helicase core of Sgs1 that disrupts Rad51 binding. In contrast to an deletion or a helicase-defective allele, this new separation-of-function allele, , does not cause DNA damage hypersensitivity or genome instability, but exhibits negative and positive genetic interactions with Δ, Δ, Δ, Δ, Δ, and Δ that are distinct from those of known mutants. Our findings suggest that the Sgs1-Rad51 interaction stimulates homologous recombination (HR). However, unlike mutations, which impair the resection of DNA double-strand ends, negative genetic interactions of the allele are not suppressed by deletion. We propose that the Sgs1-Rad51 interaction stimulates HR by facilitating the formation of the presynaptic Rad51 filament, possibly by Sgs1 competing with single-stranded DNA for replication protein A binding during resection.