Eukaryotic DNA polymerases: proposal for a revised nomenclature

DNA polymerase lambda, a novel DNA repair enzyme in human cells

DNA polymerase lambda (pol lambda) is a novel family X DNA polymerase that has been suggested to play a role in meiotic recombination and DNA repair. The recent demonstration of an intrinsic 5'-deoxyribose-5-phosphate lyase activity in pol lambda supports a function of this enzyme in base excision repair. However, the biochemical properties of the polymerization activity of this enzyme are still largely unknown. We have cloned and purified human pol lambda to homogeneity in a soluble and active form, and we present here a biochemical description of its polymerization features. In support of a role in DNA repair, pol lambda inserts nucleotides in a DNA template-dependent manner and is processive in small gaps containing a 5'-phosphate group. These properties, together with its nucleotide insertion fidelity parameters and lack of proofreading activity, indicate that pol lambda is a novel beta-like DNA polymerase. However, the high affinity of pol lambda for dNTPs (37-fold over pol beta) is consistent with its possible involvement in DNA transactions occurring under low cellular levels of dNTPs. This suggests that, despite their similarities, pol beta and pol lambda have nonredundant in vivo functions.

Reconstitution of human DNA polymerase delta using recombinant baculoviruses: the p12 subunit potentiates DNA polymerizing activity of the four-subunit enzyme

Eukaryotic DNA polymerase delta is thought to consist of three (budding yeast) or four subunits (fission yeast, mammals). Four human genes encoding polypeptides p125, p50, p66, and p12 have been assigned as subunits of DNA polymerase delta. However, rigorous purification of human or bovine DNA polymerase delta from natural sources has usually yielded two-subunit preparations containing only p125 and p50 polypeptides. To reconstitute an intact DNA polymerase delta, we have constructed recombinant baculoviruses encoding the p125, p50, p66, and p12 subunits. From insect cells infected with four baculoviruses, protein preparations containing the four polypeptides of expected sizes were isolated. The four-subunit DNA polymerase delta displayed a specific activity comparable with that of the human, bovine, and fission yeast proteins isolated from natural sources. Recombinant DNA polymerase delta efficiently replicated singly primed M13 DNA in the presence of replication protein A, proliferating cell nuclear antigen, and replication factor C and was active in the SV40 DNA replication system. A three-subunit subcomplex consisting of the p125, p50, and p66 subunits, but lacking the p12 subunit, was also isolated. The p125, p50, and p66 polypeptides formed a stable complex that displayed DNA polymerizing activity 15-fold lower than that of the four-subunit polymerase. p12, expressed and purified individually, stimulated the activity of the three-subunit complex 4-fold on poly(dA)-oligo(dT) template-primer but had no effect on the activity of the four-subunit enzyme. Therefore, the p12 subunit is required to reconstitute fully active recombinant human DNA polymerase delta.