Human DNA polymerase θ harbors DNA end-trimming activity critical for DNA repair

Cancers with hereditary defects in homologous recombination rely on DNA polymerase theta (pol theta) for repair of DNA double-strand breaks. During end joining, pol theta aligns microhomology tracts internal to 5'-resected broken ends. An unidentified nuclease trims the 3' ends before synthesis can occur. Here we report that a nuclease activity, which differs from the proofreading activity often associated with DNA polymerases, is intrinsic to the polymerase domain of pol theta. Like the DNA synthesis activity, the nuclease activity requires conserved metal-binding residues, metal ions, and dNTPs and is inhibited by ddNTPs or chain-terminated DNA. Our data indicate that pol theta repurposes metal ions in the polymerase active site for endonucleolytic cleavage and that the polymerase-active and end-trimming conformations of the enzyme are distinct. We reveal a nimble strategy of substrate processing that allows pol theta to trim or extend DNA depending on the DNA repair context.

Automated summary

The study reveals that DNA polymerase theta (pol theta) possesses a nuclease activity, distinct from its DNA synthesis activity, which plays a crucial role in the repair of DNA double-strand breaks. This nuclease activity requires metal ions, specific DNA nucleotides, and can trim or extend DNA based on the DNA repair context.