title: N-ethyl-N-nitrosourea induces A:T to C:G transversion mutations as well as transition mutations in SOS-induced Escherichia coli.
abstract: 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.
authors.affiliation: McArdle Laboratory for Cancer Research, University of Wisconsin, Madison 53706.
authors.forename: K A
authors.initials: KA
authors.lastname: Eckert
authors.forename: C A
authors.initials: CA
authors.lastname: Ingle
authors.forename: N R
authors.initials: NR
authors.lastname: Drinkwater
mesh_terms: D000225:Adenine; D001482:Base Composition / Q000187:drug effects; D001483:Base Sequence; D002872:Chromosome Deletion; D003596:Cytosine; D004249:DNA Damage; D004260:DNA Repair; D004269:DNA, Bacterial / Q000187:drug effects / Q000235:genetics*; D004926:Escherichia coli / Q000187:drug effects / Q000235:genetics*; D005038:Ethylnitrosourea / Q000494:pharmacology*; D005798:Genes, Bacterial / Q000187:drug effects; D006147:Guanine; D008969:Molecular Sequence Data; D009154:Mutation; D013014:SOS Response, Genetics; D013941:Thymine
chemical_list: D004269:DNA, Bacterial; D006147:Guanine; D003596:Cytosine; D000225:Adenine; D005038:Ethylnitrosourea; D013941:Thymine