Differences between the mutational consequences of replication of cis- and trans-opened benzo[a]pyrene 7,8-diol 9,10-epoxide- deoxyguanosine adducts in M13mp7L2 constructs

The four adducts at N-2 Of deoxyguanosine derived from cis- opening at C-10 of four optically active isomers of 7,8- dihydroxy-9, 10-epoxy-7,8,9,10-tetrahydrobenzo [a]pyrene were incorporated into 5'-TTCGAATCCTTCCCCC [context III(G)] and 5'- GGGGTTCCCGAGCGGC [context IV(G)I at the underlined site. The mutagenic consequences of these lesions in each of the two sequence contexts were examined after ligation of the modified oligonucleotides into single-stranded M13mp7L2 and replication of the vector in SOS-induced Escherichia coli. Total frequencies of base substitution mutations ranged between 14 and 48%. The mutation frequencies were generally higher in context IV(G) than in context III(G), and consisted mainly of G -->T followed by G -->C base substitutions. A substantial number of deletions or insertions of one guanine was also found for all adducts in context IV(G), where the adduct is located at the S'-end of a run of five guanines. The overall frequencies of base substitution mutations induced by cis- opened adducts were substantially higher than those observed with the trans-opened dGuo adducts in the same sequences [Page et al. (1998) Biochemistry 37, 9127-9137]. Although G-T base substitutions predominated for both the cis- and trans-opened adducts, the cis-opened dGuo adducts generally resulted in a higher proportion of G -->C [particularly in context III(G)] relative to G -->A, whereas the opposite was true for the trans-opened dGuo adducts. The present results along with previous data indicate that mutagenicity is highly dependent on a combination of sequence context and adduct stereochemistry.

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