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

Human DNA polymerase iota (pol iota) is a recently discovered enzyme that e xhibits extremely low fidelity on undamaged DNA templates. Here, we show th at pol iota is able to facilitate limited translesion replication of a thym ine-thymine cyclobutane pyrimidine dimer (CPD), More importantly, however, the bypass event is highly erroneous. Gel kinetic assays reveal that pol io ta 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 th e T T mispair significantly, the G T mispair is extended and the lesion com pletely bypassed, with the same efficiency as that of the correctly paired A T base pair. By comparison, pol iota readily misinserts two bases opposit e a 6-4 thymine-thymine pyrimidine-pyrimidone photoproduct (6-4PP), but com plete lesion bypass is only a fraction of that observed with the CPD. Our d ata indicate, therefore, that pol iota possesses the ability to insert nucl eotides opposite UV photoproducts as well as to perform unassisted transles ion replication that is likely to be highly mutagenic.