Lesion Bypass by Human DNA Polymerase μ Reveals a Template-dependent, Sequence-independent Nucleotidyl Transferase Activity

DNA polymerase μ (pol μ), which is related to terminal deoxynucleotidyl transferase and DNA polymerase β, is thought to be involved in non-homologous end joining and V(D)J recombination. Pol μ is induced by ionizing radiation and exhibits low fidelity. Analysis of translesion replication by purified human pol μ revealed that it bypasses a synthetic abasic site with high efficiency, using primarily a misalignment mechanism. It can also replicate across two tandem abasic sites, using the same mechanism. Pol μ extends primers whose 3′-terminal nucleotides are located opposite the abasic site. Most remarkably, this extension occurs via a mode of nucleotidyl transferase activity, which does not depend on the sequence of the template. This is not due to simple terminal nucleotidyl transferase activity, because pol μ is unable to add dNTPs to an oligo(dT)₂₉ primer or to a blunt end duplex oligonucleotide under standard conditions. Thus, pol μ is a dual mode DNA-synthesizing enzyme, which can act as either a classical DNA polymerase or as a non-canonical, template-dependent, but sequence-independent nucleotidyl transferase. To our knowledge, this is the first report on a DNA-synthesizing enzyme with such properties. These activities may be required for its function in non-homologous end joining in the processing of DNA ends prior to ligation.