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 co mmon base lesions produced by cellular metabolism as well as ionizing radia tion and environmental carcinogens. Escherichia coli DNA glycosylase, endon uclease III and endonuclease Vm recognize and remove these lesions from DNA , In this study, we assessed the mutagenic potential of these lesions in th e supF gene as a forward mutation target in double-stranded plasmid DNA usi ng an E. coli strain deficient in both endonuclease III and endonuclease VI II. These lesions were introduced into pTN89 DNA by the chemical oxidant os mium tetroxide. Spontaneous supF mutations occurred at a frequency of 3.03 x 10(-7) and osmium tetroxide-induced at a frequency of 8.25 x 10(-7). Sequ ence 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 domi nantly detected in the spontaneous and osmium tetroxide-induced mutations i n 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 principa l mutagenic lesions.