Sunlight mutagenesis: changes in mutational specificity during the irradiation of phage M13mp2

We reported previously that the mutations in phage M13mp2, a single-strande d DNA phage, induced by sunlight exposure are predominated by G-to-C transv ersions. We have now made an unexpected observation that an exposure to sun light for a short period of time results in induction mainly of C-to-T tran sitions while a longer exposure results in the induction of G-to-C transver sions. This peculiar phenomenon suggests that DNA damage formed by initial sunlight exposure can be transformed during an elongated exposure. 7,8-Dihy dro-8-oxoguanine (8-oxoG) in DNA might be involved in the shift of the muta tional specificity, as 8-oxoG was formed in the phage DNA upon the sunlight exposure, We also compared the mutagenic activity of UVB irradiation with that of sunlight exposure. The results demonstrate that the genotoxic prope rties of sunlight and UVB in phage M13mp2 mutagenesis are different. The sh ift in the mutational specificity associated with the dose of the sunlight may call for general cautions in the studies of agent-induced mutagenesis. (C) 1999 Elsevier Science B.V. All rights reserved.