We have determined the spectrum of base-pair substitution mutations induced in the lad gene of a uvrB-strain of Escherichia coli by two polycyclic aromatic hydrocarbons-(±)7a,8fi-dihydroxy-9p, 10I3-epoxy-7,8,9,10 tetrahydrobenzo-[a]pyrene (BPDE), and 3,4-epoxycydopenta [cd]pyrene (CPPE). Approximately 10% of all lad mutations induced by either BPDE or CPPE are nonsense mutations, suggesting that base-pair substitutions are a large fraction of the mutational events induced by these agents in the uvrB-bacteria. Both carcinogens specifically induced the G-C -* TA and, to a lesser extent, the A-T -T-A transversions. One possible mechanism for transversion induction at G-C sites by BPDE might involve carcinogen binding to the exocyclic amino group of guanine in the template strand followed by a rotation of the modified base around its glycosylic bond from the anti to the syn conformation. This could allow specific pairing of modified bases with an imino tautomer of adenine.Although the mutagenicity of most carcinogens is well documented (1), little is known about the spectrum of mutagenic events that carcinogens are capable of inducing. The most widely utilized mutagenicity test, the Ames test (1), detects the ability of chemicals to revert any one of three different point mutations (two frameshift mutations and a base-substitution mutation) but does not yet provide information about the specific changes in DNA sequence that are induced at these loci. Because there is a wide range of possible base sequence alterations it seems of fundamental importance to document the spectrum of mutations induced in vivo by carcinogens. Miller and his colleagues have developed a genetic system in Escherichia coli which is useful for rapidly and rigorously identifying base-pair substitutions that generate nonsense mutations at 64 sites within the lad gene (2). This system detects all possible base-pair substitution events with the exception of the A-T -> G-C transition. We have begun to use the lad system to explore the spectrum of base-pair substitutions induced in vivo by carcinogens.In the study reported here we focused on two polycyclic aromatic hydrocarbons which are environmental carcinogens: benzo[a]pyrene (BP) and cyclopenta[cd]pyrene (CPP) (3-5).Many carcinogens are metabolized to highly reactive species which bind covalently to DNA (6) and induce mutations in bacteria or in mammalian cells (7,8). The reactive metabolite of BP that accounts for most of the binding to DNA is a trans diol epoxide, (±)7a,8,3dihydroxy-9,, 10,3epoxy-7,8,9, 10-tetrahydrobenzo[a]pyrene (BPDE) (9). picted in Fig. 1 (10). Adducts of the trans diol epoxide with the exocyclic N6 amino group of adenine (11) and with cytosine (12) have also been reported. There is also indirect evidence that an adduct may be formed between the trans diol epoxide and the N7 position of guanine (13). CPP adducts to DNA have not been identified. Which of the BPDE-DNA adducts represent biologically significant premutational lesions? The mutational spectrum induced b...