MOLECULAR ANALYSIS OF MUTATIONS INDUCED BY ETHYLATING N-NITROSO COMPOUNDS IN THE LACI GENE OF ESCHERICHIA-COLI

We have investigated the mutational specificities of two ethylating ag ents, N-nitroso-N,N-diethylamine (NDEA) and N-ethyl-N'-nitro-N-nitroso guanidine (ENNG) in the NC+ domain of the lacI gene in Escherichia col i. G:C --> A:T transitions predominated in each of the two spectra, bu t its importance varies. While 95% of mutations induced by ENNG are G: C --> A:T transition, they account for only 61% of the total events fo llowing NDEA treatment A:T --> G:C transitions are more often recovere d following treatment with NDEA than with ENNG. Deletions, duplication s, and frameshifts were also recovered after treatment with NDEA, but not ENNG. Mutations obtained in this study are also compared with thos e induced by corresponding methylating agents. The 5' flanking base ap pears to affect the distribution of the G:C --> A:T events. G:C --> A: T transition appears more likely to be recovered at 5'-PuG-3' sites. N DEA, however, induced a significant number of G:C --> A:T changes at 5 -PuC-3' sites (51%). The possible reasons for these site-specificities are discussed. Although these alkylating agents most likely induce th e bulk of their mutation via common mutagenic intermediates, each agen t does induce a characteristic spectrum in which specific sites are en hanced or reduced.