Nej1 recruits the Srs2 helicase to DNA double-strand breaks and supports repair by a single-strand annealing-like mechanism.

Double-strand breaks (DSBs) represent the most severe DNA lesion a cell can suffer, as they pose the risk of inducing loss of genomic integrity and promote oncogenesis in mammals. Two pathways repair DSBs, nonhomologous end joining (NHEJ) and homologous recombination (HR). With respect to mechanism and genetic requirements, characterization of these pathways has revealed a large degree of functional separation between the two. Nej1 is a cell-type specific regulator essential to NHEJ in Saccharomyces cerevisiae. Srs2 is a DNA helicase with multiple roles in HR. In this study, we show that Nej1 physically interacts with Srs2. Furthermore, mutational analysis of Nej1 suggests that the interaction was strengthened by Dun1-dependent phosphorylation of Nej1 serines 297/298. Srs2 was previously shown to be recruited to replication forks, where it promotes translesion DNA synthesis. We demonstrate that Srs2 was also efficiently recruited to DSBs generated by the HO endonuclease. Additionally, efficient Srs2 recruitment to this DSB was dependent on Nej1, but independent of mechanisms facilitating Srs2 recruitment to replication forks. Functionally, both Nej1 and Srs2 were required for efficient repair of DSBs with 15-bp overhangs, a repair event reminiscent of a specific type of HR called single-strand annealing (SSA). Moreover, absence of Rad51 suppressed the SSA-defect in srs2 and nej1 strains. We suggest a model in which Nej1 recruits Srs2 to DSBs to promote NHEJ/SSA-like repair by dismantling inappropriately formed Rad51 nucleoprotein filaments. This unexpected link between NHEJ and HR components may represent cross-talk between DSB repair pathways to ensure efficient repair.