Different mutational profiles induced by N‐nitroso‐N‐ethylurea: Effects of dose and error‐prone DNA repair and correlations with DNA adducts

The DNA adducts and mutational profile produced by N‐nitroso‐N‐ethylurea (ENU) in Salmonella are examined. The adduct profile produced by ENU in isolated DNA and at two doses in Salmonella were similar, with one exception: O⁶‐ethylguanine (O⁶‐EtG) was not detected at the low dose in Salmonella. This adduct was presumably repaired by a constitutive repair system. The premutagenic adducts, O²‐ethylthymidine (O²‐EtdT) and O⁴‐ethylthymidine (O⁴‐EtdT) were detected, with the former adduct present at higher levels. The mutational profile was also determined at the same doses by utilizing a system involving a series of histidine auxotrophs of Salmonella with differing mutagenic specificities and a further subclassification of the revertants. Four different patterns of mutagenesis were observed; these were dependent on dose and on the presence or absence of the plasmid pKM101. The mutational spectrum produced at the higher dose in strains without the plasmid consisted mainly of GC→AT transitions. At the high dose, in strains harboring pKM101, three base changes contributed importantly to the mutational spectrum: GC→AT, AT→GC, and AT→CG. At the low dose in the strains without pKM101, little mutagenesis was observed, and in strains containing pKM101, mutagenesis was greatly enhanced with the most frequent mutations resulting from AT→GC and AT→CG base changes. O⁶‐EtG was presumably responsible for the bulk of the GC→AT transitions at the high dose. Calculations and evidence are presented indicating that O²‐EtdT is responsible for at least some of the mutagenesis that occurs at AT base pairs. O⁴‐EtdT and O²EtdT are probably responsible for a major fraction of the AT→GC transitions, and we suggest that error‐prone repair activity acting on O²‐EtdT and/or 04‐EtdT results in the AT→CG transversions.