Factors determining mutagenic potential for individual cis and trans opened benzo[c]phenanthrene diol...

Four adducts that would result from trans opening at C-1 of benzo[c]phenanthrene 3,4-diol 1,2-epoxide (B[c]PhDE) isomers (i.e., DE-1 enantiomers, where the epoxide oxygen and benzylic hydroxyl group are cis, and DE-2 enantiomers, where they are trans) by the N(6)-amino group of dAdo, together with the two cis opened N(6)-dAdo adducts of B[c]PhDE-1, were incorporated into two oligonucleotides at the underlined site in 5'-TTTAGAGTCTGCTCCC [context I(A)] and 5'-CAGATTTAGAGTCTGC [context II(A)]. After ligation of these, and the corresponding unsubstituted oligonucleotides, into single-stranded M13mp7L2 bacteriophage and transfection into SOS-induced Escherichia coli SMH77, base substitution mutations induced by the different B[c]PhDE-dAdo adducts were determined. These findings were compared with data [Pontén et al. (1999) Biochemistry 38, 1144-1152] for cis opened B[c]PhDE-2-dAdo adducts in the same sequence contexts. In most cases, adducts with S absolute configuration at the site of attachment of the nucleoside to the hydrocarbon had higher mutation frequencies (1.9-56.5%) than the corresponding adducts with R configuration (0.05-5.6%). For adducts derived from B[c]PhDE-1, the predominant mutations were A-->T transversions in context I(A) and A-->G transitions for most of these adducts in context II(A). For adducts derived from B[c]PhDE-2, A-->T base substitutions predominated for most of the trans adducts, but A-->G mutations were favored by the cis adduct with S configuration in either context. Thus, the structural feature that most dramatically affected mutagenic activity was the configuration of the carbon at the attachment point, with S configuration mostly being associated with greater mutagenicity than the R configuration. However, other structural variations and sequence context also affected mutagenicity, indicating that a combination of structure and context effects define mutagenicity.