A cell-free system for mitotically-stimulated TMEJ

Double-stranded DNA breaks (DSBs) are cytotoxic and must be repaired to avoid cell death or mutagenesis. DSBs that form in the S or G2 phases of the cell cycle undergo end resection, creating substrates for homologous recombination (HR) and alternative end joining pathways. Theta-mediated end joining (TMEJ) is a DNA polymerase θ (Pol θ)-dependent repair pathway that joins resected DSB ends by annealing short regions of microhomology. TMEJ becomes particularly important when HR is compromised or during mitosis. Despite intensive investigation, the mechanism of TMEJ remains poorly understood, in part due to the lack of an efficient cell-free system that recapitulates this reaction. Here, we address this deficit using Xenopus egg extracts. We show that a linear DNA template containing a 3 ^' single-stranded DNA overhang with terminal microhomology undergoes efficient intermolecular joining in nucleoplasmic extracts derived from Xenopus eggs. Joining is abolished upon Pol θ depletion, and restoration of joining requires Pol θ that is proficient in both ATPase and DNA polymerase activities. Furthermore, joining is accelerated by recombinant Cyclin B-CDK1, which also enhances the recruitment of Pol θ to single-stranded DNA ends. This assay provides a biochemically tractable approach for dissecting the mechanism and regulation of TMEJ in a physiologically complex environment.