Specificity of ionizing radiation–induced mutagenesis in thelacregion of single–stranded phage M13 mp10 DNA

M13 mp10 single-stranded phage DNA was irradiated with 60 Co gamma-rays, and transfected into Escherichia coli. One hundred and sixteen mutant clones having lesions in the lac insert were selected, and mutational sites were examined by DNA sequence analysis. Fourteen out of the 15 nucleotide changes thus detected were base substitutions, and the rest was a base addition. Transitions and transversions were almost equal in number. Mutational events were observed at cytosine residues more frequently than at other residues, and the predominant base change was a C ---- T transition. Possible roles in gamma-ray-induced mutagenesis played by the misincorporation of dAMP owing to radiolytic derivatives of cytosine residues and/or formation of apurinic/apyrimidinic sites are discussed.