Effects of a guanine-derived formamidopyrimidine lesion on DNA replication. Translesion DNA synthesis, nucleotide insertion, and extension kinetics

2,6-Diamino-4-hydroxy-5-formamidopyrimidine derived from guanine (FapyG) is a major DNA lesion formed by reactive oxygen species. In this study, a defined oligonucleotide template containing a 5-N-methylated analog of FapyG (mFapyG) was prepared, and its effect on DNA replication was quantitatively assessed in vitro. The results were further compared with those obtained for 7,8-dihydro-8-oxoguanine and an apurinic/apyrimidinic site embedded in the same sequence context. mFapyG constituted a fairly strong but not absolute block to DNA synthesis catalyzed by Escherichia coli DNA polymerase I Klenow fragment with and without an associated 3′-5′ exonuclease activity, thereby permitting translesion synthesis with a limited efficiency. The efficiency of translesion synthesis was G > 7,8-dihydro-8-oxoguanine > mFapyG > apurinic/apyrimidinic site. Analysis of the nucleotide insertion (f_ins= V_max/K_m for insertion) and extension (f_ext = V_max/K_m for extension) efficiencies for mFapyG revealed that the extension step constituted a major kinetic barrier to DNA synthesis. When mFapyG was bypassed, dCMP, a cognate nucleotide, was preferentially inserted opposite the lesion (dCMP (relative f_ins = 1) ≫ dTMP (2.4 × 10^-4) ≈ dAMP (8.1 × 10^-5) > dGMP (4.5 × 10^-7)), and the primer terminus containing a mFapyG:C pair was most efficiently extended (mFapyG:C (relative f_ext = 1) > mFapyG:T (4.6 × 10^-3) ≫ mFapyG:A and mFapyG:G (extension not observed)). Thus, mFapyG is a potentially lethal but not premutagenic lesion.