Misinsertion and bypass of thymine-thymine dimers by human DNA polymerase ι

Human DNA polymerase iota (pol iota) is a recently discovered enzyme that exhibits extremely low fidelity on undamaged DNA templates. Here, we show that pol iota is able to facilitate limited translesion replication of a thymine-thymine cyclobutane pyrimidine dimer (CPD), More importantly, however, the bypass event is highly erroneous. Gel kinetic assays reveal that pol iota misinserts T or G opposite the 3' T of the CPD similar to 1.5 times more frequently than the correct base, A. While pol iota is unable to extend the T T mispair significantly, the G T mispair is extended and the lesion completely bypassed, with the same efficiency as that of the correctly paired A T base pair. By comparison, pol iota readily misinserts two bases opposite a 6-4 thymine-thymine pyrimidine-pyrimidone photoproduct (6-4PP), but complete lesion bypass is only a fraction of that observed with the CPD. Our data indicate, therefore, that pol iota possesses the ability to insert nucleotides opposite UV photoproducts as well as to perform unassisted translesion replication that is likely to be highly mutagenic.