Translesion synthesis across O6-alkylguanine DNA adducts by recombinant human DNA polymerases

Previous studies have shown that replicative bacterial and viral DNA polymerases are able to bypass the mutagenic lesions O-6-methyl and -benzyl (Bz) G. Recombinant human polymerase (pol) delta also copied past these two lesions but was totally blocked by O-6-[4-oxo-4-(3-pyridyl) butyl] (Pob)G, an important mutagenic lesion formed following metabolic activation of the tobacco-specific carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone. The human translesion pols iota and kappa produced mainly only 1-base incorporation opposite O-6-MeG and O-6-BzG and had very low activity in copying O-6-PobG. Human pol eta copied past all three adducts. Steady-state kinetic analysis showed similar efficiencies of insertion opposite the O-6-alkylG adducts for dCTP and dTTP with pol eta and kappa; pol iota showed a strong preference for dTTP. pol eta, iota, and kappa showed pre-steady-state kinetic bursts for dCTP incorporation opposite G and O-6-MeG but little, if any, for O-6-BzG or O-6-PobG. Analysis of the pol eta O-6-PobG products indicated that the insertion of G was opposite the base (C) 5' of the adduct, but this product was not extended. Mass spectrometry analysis of all of the pol eta primer extension products indicated multiple components, mainly with C or T inserted opposite O-6-alkylG but with no deletions in the cases of O-6-MeG and O-6-PobG. With pol eta and O-6-BzG, products were also obtained with -1 and -2 deletions and also with A inserted (opposite O-6-BzG). The results with pol eta may be relevant to some mutations previously reported with O-6-alkylG adducts in mammalian cells.