Sequence context profoundly influences the mutagenic potency of trans-opened benzo[a]pyrene 7,8-diol 9,10-epoxide-purine nucleoside adducts in site-specific mutation studies.

The postoligomerization method was used to prepare oligonucleotide 16-mers that contained dAdo or dGuo adducts, derived from trans opening of each enantiomer of the two diastereomeric benzo[a]pyrene 7,8-diol 9,10-epoxides, in two sequence contexts. These 16 oligonucleotides, along with the four corresponding oligonucleotides containing unsubstituted purines, were ligated into single-stranded DNA from bacteriophage M13mp7L2 and transfected into Escherichia coli SMH77. The mutagenic effects of replication past these adducts were then evaluated. The various adduct isomers induced point mutations at different frequencies and with different distributions of mutation types, as was anticipated. However, sequence context had the most substantial effects on mutation frequency. A high frequency of deletions of a single guanine was found in a context where the dGuo adduct was at the 3'-end of a run of five guanines, whereas no single base deletion was found in the other context studied, 5'-CGA-3'. Mutation frequencies in constructs containing dAdo adducts were much higher in a 5'-TAG-3' context (37-58%, depending on the individual isomer) than in a 5'-GAT-3' context (5-20%), and for a given adduct, mutation frequency was up to 10-fold higher in the former sequence than in the latter. These findings indicate that sequence context effects need more thorough evaluation if the goal of understanding the mechanism through which DNA adducts lead to mutation is to be achieved.