DNA damage caused by the binding of the tumorigen 7R,8S-diol 9S,10R-epoxide 
(B[a]PDE), a metabolite of bezo[a]pyrene, to guanine in CpG dinucleotide 
sequences could affect DNA methylation and, thus, represent a potential 
epigenetic mechanism of chemical carcinogenesis. In this work, we investigated 
the impact of stereoisomeric (+)- and (-)-trans-anti-B[a]P-N(2)-dG adducts (B(+) 
and B(-)) on DNA methylation by prokaryotic DNA methyltransferases M.SssI and 
M.HhaI. These two methyltransferases recognize CpG and GCGC sequences, 
respectively, and transfer a methyl group to the C5 atom of cytosine (C). A 
series of 18-mer unmethylated or hemimethylated oligodeoxynucleotide duplexes 
containing trans-anti-B[a]P-N(2)-dG adducts was generated. The B(+) or B(-) 
residues were introduced either 5' or 3' adjacent or opposite to the target 
2'-deoxycytidines. The B[a]PDE lesions practically produced no effect on M.SssI 
binding to DNA but reduced M.HhaI binding by 1-2 orders of magnitude. In most 
cases, the benzo[a]pyrenyl residues decreased the methylation efficiency of 
hemimethylated and unmethylated DNA by M.SssI and M.HhaI. An absence of the 
methylation of hemimethylated duplexes was observed when either the (+)- or the 
(-)-trans-anti-B[a]P-N(2)-dG adduct was positioned 5' to the target dC. The 
effects observed may be related to the minor groove conformation of the bulky 
benzo[a]pyrenyl residue and to a perturbation of the normal contacts of the 
methyltransferase catalytic loop with the B[a]PDE-modified DNA. Our results 
indicate that a trans-anti-B[a]P-N(2)-dG lesion flanking a target dC in the CpG 
dinucleotide sequence on its 5'-side has a greater adverse impact on methylation 
than the same lesion when it is 3' adjacent or opposite to the target dC.