Effect of benzo-ring hydroxyl groups on site-specific mutagenesis by tetrahydrobenzo[a]pyrene adducts at N(6) of deoxyadenosine

We have previously investigated the mutations induced on replication in Escherichia coli of the M13mp7L2 genome containing each of the eight possible adducts derived from the four optically active 7,8-diol 9,10-epoxide metabolites of benzo[a]pyrene (B[a]P) by alkylation of a specific deoxyadenosine (dAdo) residue at N(6). Observed mutational frequencies depended in part on the relative spatial orientations of the three hydroxyl groups in these adducts. To determine how the presence or absence of these hydroxyl groups affects mutational response, we have synthesized 16-mer oligonucleotides with the same sequence as one of those previously studied with the diol epoxide adducts, but containing B[a]P-dAdo adducts in which two or all three of the adduct hydroxyl groups were replaced by hydrogen. Transfection of the adducted M13 constructs into SOS-induced Escherichia coli consistently gave fewer infective centers than the control construct, with viabilities ranging from 8.4 to 44.9% relative to control. In general, decreasing the number of adduct hydroxyls decreased the total frequency of substitution mutations induced. For all but one of the present adducts, the total mutational frequency was lower than that for any of the previously reported diol epoxide adducts in the same sequence. Remarkably, this (9S,10R)-adduct with cis orientation of the dAdo residue and the 9-OH group gave the highest mutational frequency of all the B[a]P adducts studied in this sequence, including the diol epoxide adducts. With the present adducts, A --> T transversions predominated, with smaller numbers of A --> G transitions and even fewer A --> C transversions.