DNA polymerase theta (POLQ) can extend from mismatches and from bases opposite a (6-4) photoproduct.

DNA polymerase theta (pol theta) is a nuclear A-family DNA polymerase encoded by the POLQ gene in vertebrate cells. The biochemical properties of pol theta and of Polq-defective mice have suggested that pol theta participates in DNA damage tolerance. For example, pol theta was previously found to be proficient not only in incorporation of a nucleotide opposite a thymine glycol or an abasic site, but also extends a polynucleotide chain efficiently from the base opposite the lesion. We carried out experiments to determine whether this ability to extend from non-standard termini is a more general property of the enzyme. Pol theta extended relatively efficiently from matched termini as well as termini with A:G, A:T and A:C mismatches, with less descrimination than a well-studied A-family DNA polymerase, exonuclease-free pol I from E. coli. Although pol theta was unable to, by itself, bypass a cyclobutane pyrimidine dimer or a (6-4) photoproduct, it could perform some extension from primers with bases placed across from these lesions. When pol theta was combined with DNA polymerase iota, an enzyme that can insert a base opposite a UV-induced (6-4) photoproduct, complete bypass of a (6-4) photoproduct was possible. These data show that in addition to its ability to insert nucleotides opposite some DNA lesions, pol theta is proficient at extension of unpaired termini. These results show the potential of pol theta to act as an extender after incorporation of nucleotides by other DNA polymerases, and aid in understanding the role of pol theta in somatic mutagenesis and genome instability.