Mutation spectra of N-ethyl-N′-nitro-N-nitrosoguanidine and 1-(2-hydroxyethyl)-1-nitrosourea in Escherichia coli

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Abstract
DNA sequencing was used to determine the specific types of DNA base changes induced following in vivo exposure of Escherichia coli to the ethylating agent N-ethyl-N′-nitro-N-nitrosoguanidine (ENNG) and the hydroxyethylating agent 1-(2-hydroxyethyl)-1-nitrosourea (HENU) using the xanthine guanine phosphoribosyltransferase (gpt) gene as the genetic target. We observed that 22/30 of the ENNG-induced mutations were GC→AT transitions, 4/30 were AT→GC transitions, 3/30 were AT→TA transversions, and 1/30 was an AT→CG transversion. We observed that 37/40 HENU-induced mutations were GC→AT transitions and that the remaining 3/40 were AT→GC transitions. A majority of the GC→AT transitions induced by ENNG and HENU (68% and 73%, respectively) occurred at the second guanine of the sequence 5′-GG(A or T)-3′; this sequence specificity was similar to that previously seen with the alkylating agents N-methyl- and N-ethyl-N-nitrosourea (MNU and ENU) and N-methyl-N′-nitro-N-nitrosoguanidine (MNNG). A DNA strand preference for the G→A changes (antisense strand), previously noted for MNU, ENU, and MNNG, was observed following exposure to HENU and ENNG. The AT→GC transitions induced by ENNG, HENU, and ENU also exhibit a sequence specificity with 13/13 mutations occurring at the T of the sequence 5′-NTC-3′. A strand preference was not apparent for these mutations.