Spontaneous and osmium tetroxide-induced mutagenesis in an Escherichia coli strain deficient in both endonuclease III and endonuclease VIII

Thymine glycol, uracil glycol, 5-hydroxycytosine and 5-hydroxyuracil are common base lesions produced by cellular metabolism as well as ionizing radiation and environmental carcinogens. Escherichia coli DNA glycosylase, endonuclease III and endonuclease VIII recognize and remove these lesions from DNA. In this study, we assessed the mutagenic potential of these lesions in the supF gene as a forward mutation target in double-stranded plasmid DNA using an E.coli strain deficient in both endonuclease III and endonuclease VIII. These lesions were introduced into pTN89 DNA by the chemical oxidant osmium tetroxide. Spontaneous supF mutations occurred at a frequency of 3.03×10–7 and osmium tetroxide-induced at a frequency of 8.25×10–7. Sequence analysis of supF mutants revealed that mutations occurred at cytosine sites rather than thymine sites, suggesting that thymine glycol is not the principal premutagenic lesion. In contrast, G:C→A:T transitions were dominantly detected in the spontaneous and osmium tetroxide-induced mutations in the endonuclease III and endonuclease VIII double defective host. In this case, products of cytosine oxidation such as 5-hydroxycytosine, which are the substrate for endonuclease III and endonuclease VIII, were the principal mutagenic lesions.