The stereochemistry of benzo[a]pyrene-2′-deoxyguanosine adducts affects DNA methylation by SssI and HhaI DNA methyltransferases

The biologically most significant genotoxic metabolite of the environmental pollutant benzo[a]pyrene (B[a]P), (+)-7R,8S-diol 9S,10R-epoxide, reacts chemically with guanine in DNA, resulting in the predominant formation of (+)-trans-B[a]P-N²-dG and, to a lesser extent, (+)-cis-B[a]P-N ²-dG adducts. Here, we compare the effects of the adduct stereochemistry and conformation on the methylation of cytosine catalyzed by two purified prokaryotic DNA methyltransferases (MTases), SssI and HhaI, with the lesions positioned within or adjacent to their CG and GCGC recognition sites, respectively. The fluorescence properties of the pyrenyl residues of the (+)-cis-B[a]P-N²-dG and (+)-trans-B[a]P-N²-dG adducts in complexes with MTases are enhanced, but to different extents, indicating that aromatic B[a]P residues are positioned in different microenvironments in the DNA-protein complexes. We have previously shown that the (+)-trans-isomeric adduct inhibits both the binding and methylating efficiencies (k_cat) of both MTases [Subach OM, Baskunov VB, Darii MV, Maltseva DV, Alexandrov DA, Kirsanova OV, Kolbanovskiy A, Kolbanovskiy M, Johnson F, Bonala R, et al. (2006) Biochemistry45, 6142-6159]. Here we show that the stereoisomeric (+)-cis-B[a]P-N²-dG lesion has only a minimal effect on the binding of these MTases and on k_cat. The minor-groove (+)-trans adduct interferes with the formation of the normal DNA minor-groove contacts with the catalytic loop of the MTases. However, the intercalated base-displaced (+)-cis adduct does not interfere with the minor-groove DNA-catalytic loop contacts, allowing near-normal binding of the MTases and undiminished k_cat values.