Overexpression of Ogt reduces MNU and ENU induced transition, but not transversion, mutations in E. coli

Abstract

Studies of alkylation-induced mutations in Escherichia coli FX-11 revealed that both N-ethyl-N-nitrosourea (ENU) and N-methyl-N-nitrosourea (MNU) produced tRNA suppressor mutations (G:C to A:T) but only ENU produced a significant number of backmutations (A:T to G:C, A:T to T:A and A:T to C:G). Further, the ENU-induced transversions were absent in a UmuC-defective strain. This suggested that transition mutations could result from alkylation of guanine or thymine at the O6- and O4-positions, respectively, but that transversions might result from alkylation of thymine at the O2-position. To test this idea, the gene encoding O6-alkylguanine-DNA methyltransferase (ogt) was recombined into a plasmid to overexpress the cellular levels of this enzyme. Ogt protein can de-alkylate O6-alkylguanine and O4-alkylthymine, but not O2-alkylthymine. Cells harboring the plasmid (or a control plasmid lacking the ogt gene) were exposed to different concentrations of MNU or ENU and the resulting mutations were analyzed. With either MNU or ENU, the frequency of GlnVo suppressors was reduced about 70-fold in the Ogt-overexpressing cells, suggesting that Ogt eliminated O6-alkylguanine. Similarly, GlnUo suppressor frequencies were substantially reduced. In contrast, the reduction in frequency for the backmutations was slight, only about 2.5-fold with MNU and less than two-fold for ENU. However, DNA sequence analysis of the backmutations showed that only A:T to G:C transitions were affected by overexpression of Ogt, suggesting repair of O4-alkylthymine. The frequency of transversions, in comparison, was essentially unaltered. These results implicate O2-alkylthymine as a likely candidate for transversion mutagenesis induced by ENU.