N-ethyl-N-nitrosourea induces A:T to C:G transversion mutations as well as transition mutations in SOS-induced Escherichia coli

The fixation of DNA lesions induced in Escherichia coli by N-ethyl-N-nitrosourea (ENU) occurs by both SOS-dependent and SOS-independent pathways. To determine whether these pathways result in differential processing of ENU-induced lesions, we have analyzed the DNA sequence changes of mutations induced at a plasmid-encoded herpes simplex virus type 1 thymidine kinase gene by ENU treatment of plasmid-bearing RecA- and RecA+ bacteria, and by transformation of RecA-, RecA+ and SOS-induced RecA+ bacteria with ENU-modified plasmid DNA. Transition mutations were the predominant types of base substitution mutations observed for wild-type and RecA- E. coli, consistent with the SOS-independent mispairing of O6-ethylguanine and O4-ethylthymine adducts during DNA replication. Under conditions of SOS processing of ENU lesions, however, we observed the frequent induction of A:T----C:G transversion mutations. The proportion of A:T----C:G transversion mutations (42%) observed after transformation of SOS-induced bacteria with ENU modified DNA was approximately equal to that of the G:C----A:T transitions (46%). The frequencies of these mutations were increased 20- and 5-fold respectively over that observed for non-induced RecA+ cells. We suggest that ethylated DNA lesions which normally block DNA replication can be processed to yield A:T----C:G transversion mutations in SOS-induced E. coli.