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We have shown previously that UVC irradiation of benzo[a]pyrene diol epoxide (BPDE)-adducted DNA (BPDE/UVC) induces an increase in mutation frequency in the supF gene greater than the calculated additive value derived from either treatment alone, with a greater absolute increase in the level of BPDE signature transversions. Possible explanations were that (i) the BPDE adducts are photoactivated to a more mutagenic lesion or (ii) the presence of UV-induced DNA damage enhanced the mutagenicity of BPDE adducts elsewhere on the DNA. In the present study, to determine which of these mechanisms is responsible for the enhanced mutagenicity of the combined treatment, plasmid pSP189 containing supF was treated with UVC radiation before BPDE treatment (UVC/BPDE). If BPDE adducts were being modified by UV irradiation to more mutagenic species, then reversing the order of exposure would be predicted to lower the mutation frequency and the number of transversions. Conversely, if merely the presence of UV damage influences the mutagenicity of BPDE adducts (or vice versa), the observed mutagenicity should be independent of the order of exposure. Previously, treatment with BPDE/UVC increased the mutation frequency by &gt;400% over the calculated additive value derived from the individual BPDE and UV exposures. In the present study, treatment with UVC followed by BPDE increased the mutation frequency by only approximately 60%, compared with the corresponding calculated additive value. However, whilst this shows that the order of treatment affects the mutation frequency, there was little change in the percentage of base substitutions in the two spectra. Hence, whilst the change in mutation frequency is consistent with UVC directly enhancing the mutagenicity of the BPDE adducts, the similarity in the types of mutations induced by BPDE/UVC and UVC/BPDE suggests that the mechanism may not be that simple.

Effects of the order of exposure to a binary mixture of mutagens on the induced mutation spectra in the supF gene