Single-strand annealing mediates the conservative repair of double-strand DNA breaks in homologous recombination-defective germ cells of Caenorhabditis elegans

•A missense mutation of a Rad54 homolog in C. elegans decreases ATPase activity.•Rad54 in C. elegans promotes the dissociation of Rad51 from the synapsed DNA intermediates not the prior association of Rad51to double-strand DNA breaks.•The single-strand annealing pathway actively repairs double-strand DNA breaks in the C. elegans germ cells defective in homologous recombination. A missense mutation in C. elegans RAD-54, a homolog of RAD54 that operates in the homologous recombination (HR) pathway, was found to decrease ATPase activity in vitro. The hypomorphic mutation caused hypersensitivity of C. elegans germ cells to double-strand DNA breaks (DSBs). Although the formation of RAD-51 foci at DSBs was normal in both the mutant and knockdown worms, their subsequent dissipation was slow. The rad-54-deficient phenotypes were greatly aggravated when combined with an xpf-1 mutation, suggesting a conservative role of single-strand annealing (SSA) for DSB repair in HR-defective worms. The phenotypes of doubly-deficient rad-54;xpf-1 worms were partially suppressed by a mutation of lig-4, a nonhomologous end-joining (NHEJ) factor. In summary, RAD-54 is required for the dissociation of RAD-51 from DSB sites in C. elegans germ cells. Also, NHEJ and SSA exert negative and positive effects, respectively, on genome stability when HR is defective.