Mutation spectrum in the lacI gene, induced by γ-radiation in aqueous solution under oxic conditions

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Irradiation of DNA in a cellular environment leads to many types of DNA damage, resulting from various effects of γ-radiation. One of these effects is the formation of water-derived radicals (e.g., ·OH radicals), which are formed in the vicinity of DNA (indirect effect). To study the influencence of the indirect effect on γ-radiation-induced mutations, a newly constructed plasmid, containing the lacI gene as a target gene, was irradiated with 60Co γ-radiation in aqueous solution, in the presence of oxygen. Under these circumstances, only ·OH radicals will be responsible for the induced mutations. Sequence analysis of the γ-radiation-induced mutations showed that 96% of all mutations were base pair substitutions, 87% of which occurred in the lacI gene, the others are formed in the lac operator part. All γ-radiation-induced mutations in the lacI gene occurred exclusively on G:C base pairs, and no mutations at A:T base pairs could be detected. In the spontaneous mutation spectrum, 83% of all mutations were base pair substitutions, 35% of which occured in the lacI gene and 48% in the lac operator part. Base pair substitutions on G:C base pairs were very similar in the γ-radiation-induced and in the spontaneous mutation spectrum, implying a high contribution of · OH radicals to spontaneous mutagenesis. A:T to G:C transitions accounted for 10% of all spontaneous base pair substitutions in the lacI gene and are probably the result of effects, other than just · OH radicals. It can be concluded that ·OH radicals are an important source for mutations at G:C base pairs. In this paper, the extracellular γ-radiation-induced mutation spectrum is also compared to the previously obtained, intracellular γ-radiation-induced mutation spectrum of the lacI gene. Comparison shows some differences, such as relative high amounts of mutations at A:T base pairs, G:C to T:A transversions and frameshift mutations in the intracellular γ-radiation-induced mutation spectrum, as compared to the extracellular γ-radiation-induced mutation spectrum. Since the extracellular γ-radiation-induced mutation at A:T base pairs in the intracellular γ-radiation-induced mutation spectrum are apparently the result of additional or other factors.