Role of Human DNA Polymerase κ in Extension Opposite from a cis–syn Thymine Dimer

Exposure of DNA to UV radiation causes covalent linkages between adjacent pyrimidines. The most common lesion found in DNA from these UV-induced linkages is the cis–syn cyclobutane pyrimidine dimer. Human DNA polymerase κ (Polκ), a member of the Y-family of DNA polymerases, is unable to insert nucleotides opposite the 3′T of a cis–syn T-T dimer, but it can efficiently extend from a nucleotide inserted opposite the 3′T of the dimer by another DNA polymerase. We present here the structure of human Polκ in the act of inserting a nucleotide opposite the 5′T of the cis–syn T-T dimer. The structure reveals a constrained active-site cleft that is unable to accommodate the 3′T of a cis–syn T-T dimer but is remarkably well adapted to accommodate the 5′T via Watson–Crick base pairing, in accord with a proposed role for Polκ in the extension reaction opposite from cyclobutane pyrimidine dimers in vivo.