N-Ethyl-N-nitrosourea induces A:T to C:G transversion mutations as well as transition mutations in SOS-induced Escherichiu 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.

Effect of host bacteria genotype on spontaneous reversions of Bacillus subtilis bacteriophage Φ29 sus17 nonsense codon

“…Then, the frequency of A:T-C :G transversions became approximately equal to G:C-A :T transitions [22]. Transitions were found to be the most frequent base pair changes in an E. coli plasmid in both recA 3 and recA bacteria unless the SOS functions were induced.…”

“…Transitions were found to be the most frequent base pair changes in an E. coli plasmid in both recA 3 and recA bacteria unless the SOS functions were induced. Then, the frequency of A:T-C :G transversions became approximately equal to G:C-A :T transitions [22]. Constitutive activation of RecA (recA730) increased in E. coli both transitions and transversions, but transversions were more ampli¢ed [23].…”

Frequency and Spectrum of lacI Mutations in the Liver of Big Blue Mice Following the Administration of Genotoxic Carcinogens Singly and in Series

“…Of these transversions, three were A:T → T:A, and two were G:C → T:A. ENU also induced two G:C → A:T transitions and two A:T → G:C transitions. Although the most common transversion mutation induced by ENU in vivo is A:T → T:A, ENU can induce a high percentage of A:T → C:G transversions following the induction of SOS proteins in Esherichia coli (Walker et al 1996;Eckert, Ingle, and Drinkwater 1989) These SOS enzymes are involved in the mutagenic bypass of lesions like O 4 -ethylthymine that block DNA replication (Eckert, Ingle, and Drinkwater 1989;Fix) 19930. Previous administration of B[a]P may induce a similar set of enzymes in mouse liver prior to ENU exposure and cause an increase in A:T → C:G transversions (Hanawalt 2001).…”

The Role of Mutagenic Metal Ions in Mediating In Vitro Mispairing by Alkylpyrimidines

“…Presumably, these mutations resulted from the unrepaired 06-Et-dG (2,25,26) and 04-Et-dT (2,5,27) lesions, respectively, as a consequence of their capacity to mispair with dT and dG, respectively, during DNA replication. Under SOS-induction, ENU generated a large fraction (46%) of transversion mutations at A-T base pairs in E. coli (28). In human cells, ENU produced a significant number (29% or more) of transversion mutations at the A-T base pairs (8,29) in addition to the same G-C->A-T and A-T-*GC transitions observed in E. coli.…”

“…Since (10)(11)(12)(13)(14) and humans (9,15). The reactivity of ENU allows it to form a diverse set of DNA adducts both in vitro and in vivo (4,6,16 (6,28), human cells (8,29) and animal systems (23), suggesting that dA or dT adducts and/or a breakdown product of these adducts are responsible for A-T mutations. A*T->G*C transitions can be derived from 04-Et-dT by mispairing with dG (27).…”