Mutagenesis and O6-ethylguanine levels in DNA from N-nitroso-N-ethylurea-treated Salmonella typhimurium: evidence for a high mutational efficiency of O6-ethylguanine.

The dose-responses for N-nitroso-N-ethylurea (NEU)-induced mutagenesis in the hisG46 mutant, Salmonella typhimurium TA1535, and for the formation of O6-ethylguanine (O6-EtGua) and 7-ethylguanine in the DNA isolated from these cells were measured. Mutagenesis and O6-EtGua formation exhibited threshold-like behavior, whereas the formation of 7-ethylguanine was linear with dose. These results are consistent with a dependence of mutagenesis on O6-EtGua. There was no threshold in the production of O6-EtGua in isolated DNA treated with NEU. The failure of O6-EtGua to appreciably accumulate in the cellular DNA at low doses of NEU was attributed to a saturable, constitutive repair activity in the bacteria. Based on (i) the ratio of O6-EtGua in DNA to revertant fraction, (ii) published values for the size of the Salmonella genome and (iii) the target size and target bases (guanine-cytosine base pairs) for reversion of the hisG46 (missense) mutation, it was calculated that about 1/3 of the O6-EtGua's in the DNA led to mutations. Using the same calculations and data from previous experiments, a mutational efficiency for O6-methylguanine of 2/3 was obtained. Threshold-type responses in NEU-induced mutagenesis were observed in the other hisG46 mutants, TA100 and TA1975, but not in the frameshift mutant, TA98 where the dose response was linear. As TA98 contains the same DNA repair systems as TA100, frameshift mutations induced by NEU may result from DNA adducts produced linearly with dose.