Evaluating the contribution of base stacking during translesion DNA replication.

Despite the nontemplating nature of the abasic site, dAMP is often preferentially inserted opposite the lesion, a phenomenon commonly referred to as the "A-rule". We have evaluated the molecular mechanism accounting for this unique behavior using a thorough kinetic approach to evaluate polymerization efficiency during translesion DNA replication. Using the bacteriophage T4 DNA polymerase, we have measured the insertion of a series of modified nucleotides and have demonstrated that increasing the size of the nucleobase does not correlate with increased insertion efficiency opposite an abasic site. One analogue, 5-nitroindolyl-2'-deoxyriboside triphosphate, was unique as it was inserted opposite the lesion with approximately 1000-fold greater efficiency compared to that for dAMP insertion. Pre-steady-state kinetic measurements yield a kpol value of 126 s(-1) and a Kd value of 18 microM for the insertion of 5-nitroindolyl-2'-deoxyriboside triphosphate opposite the abasic site. These values rival those associated with the enzymatic formation of a natural Watson-Crick base pair. These results not only reiterate that hydrogen bonding is not necessary for nucleotide insertion but also indicate that the base-stacking and/or desolvation capabilities of the incoming nucleobase may indeed play the predominant role in generating efficient DNA polymerization. A model accounting for the increase in catalytic efficiency of this unique nucleobase is provided and invokes pi-pi stacking interactions of the aromatic moiety of the incoming nucleobase with aromatic amino acids present in the polymerase's active site. Finally, differences in the rate of 5-nitroindolyl-2'-deoxyriboside triphosphate insertion opposite an abasic site are measured between the bacteriophage T4 DNA polymerase and the Klenow fragment. These kinetic differences are interpreted with regard to the differences in various structural components between the two enzymes and are consistent with the proposed model for DNA polymerization.