DNA polymerases η and κ are responsible for error-free translesion DNA synthesis activity over a thymine dimer in oocyte extracts

In translesion synthesis (TLS), specialized DNA polymerases (pols) facilitate progression of replication forks stalled by DNA damage. Although multiple TLS pols have been identified in eukaryotes, little is known about endogenous TLS pols and their relative contributions to TLS in vivo because of their low cellular abundance. Taking advantage of Xenopus laevis oocyte cells, with their extraordinary size and abundant enzymes involved in DNA metabolism, we have identified and characterized endogenous TLS pols for DNA damage induced by ultraviolet (UV) irradiation. We designed a TLS assay which monitors primer elongation on a synthetic oligomer template over a single UV-induced lesion, either a cys–syn cyclobutane pyrimidine dimer (CPD) or a pyrimidine (6-4) pyrimidone photoproduct. Four distinct TLS activities (TLS1–TLS4) were identified in X. laevis oocyte extracts, using three template/primer (T/P) DNA substrates having various sites at which primer extension is initiated relative to the lesion. TLS1 and TLS2 activities appear to be sequence-dependent. TLS3 and TLS4 extended the primers over the CPD in an error-free manner irrespective of sequence context. Base insertion opposite the CPD of the T/P substrate in which the 3′-end of the primer is placed one base upstream of the lesion was observed only with TLS3. TLS3 and TLS4 showed primer extension with similar efficiencies on the T/P substrate whose 3′-primer terminal dinucleotide (AA) was complementary to the CPD lesion. Investigations with antibodies and recombinant pols revealed that TLS3 and TLS4 were most likely attributable to pol η and pol κ, respectively. These results indicate that error-free insertion in CPD bypass is due mainly to pol η (TLS3) in the extracts, and suggest that pol κ (TLS4) may assist pol η (TLS3) in error-free extension during CPD bypass.