Pre-steady-state kinetic studies of the fidelity of Sulfolobus solfataricus P2 DNA polymerase IV.

Sulfolobus solfataricus P2 DNA polymerase IV (Dpo4) is a thermostable archaeal enzyme and a member of the error-prone and lesion-bypass Y-family. In this paper, for the first time, the fidelity of a Y-family polymerase, Dpo4, was determined using pre-steady-state kinetic analysis of the incorporation of a single nucleotide into an undamaged DNA substrate 21/41-mer at 37 degrees C. We assessed single-turnover (with Dpo4 in molar excess over DNA) saturation kinetics for all 16 possible nucleotide incorporations. The fidelity of Dpo4 was estimated to be in the range of 10(-3)-10(-4). Interestingly, the ground-state binding affinity of correct nucleotides (70-230 microM) is 10-50-fold weaker than those of replicative DNA polymerases. Such a low affinity is consistent with the lack of interactions between Dpo4 and the bound nucleotides as revealed in the crystal structure of Dpo4, DNA, and a matched nucleotide. The affinity of incorrect nucleotides for Dpo4 is approximately 2-10-fold weaker than that of correct nucleotides. Intriguingly, the mismatched dCTP has an affinity similar to that of the matched nucleotides when it is incorporated against a pyrimidine template base flanked by a 5'-template guanine. The incoming dCTP likely skips the first available template base and base pairs with the 5'-template guanine, as observed in the crystal structure of Dpo4, DNA, and a mismatched nucleotide. The mismatch incorporation rates, regardless of the 5'-template base, were approximately 2-3 orders of magnitude slower than the incorporation rates for matched nucleotides, which is the predominant contribution to the fidelity of Dpo4.