Influence of Neighboring Base Sequence on the Distribution and Repair of TV-Ethyl-Af-nitrosourea-induced Lesions in Escherìchiacoli1

.V-Kiliyl-.V-nitrtisouroll-induced mutations occurring within a ISO-base pair target in the lad gene of Escherìchiacoli were characterized by DNA sequencing. In total, 109 mutations were characterized in a wildtype background and 100 in an excision-repair-deficient (UvrB~) back ground. The majority of mutations induced in the two backgrounds (77 and 85%, respectively) were (.:( = >A:T transitions, presumably result ing from miscoding O'-ethylguanine lesions. A significant proportion of the mutations (17 and 15%, respectively) were A:T = >G:C transitions, which probably result from miscoding O4-ethylthymine lesions. An analy sis of the distribution of both types of mutation in the two backgrounds reveals two distinct influences of neighboring base sequence. These effects apply equally to both the G:C = >A:T and A:T = >G:C transitions. Firstly, miscoding lesions are most likely to occur at 5'-purine-G-3' or 5'-purine-T-3' sites. Secondly, the excision-repair machinery is less efficient at removing both 06-ethylguanine and O'-ethylthymine lesions which are flanked on both sides by G:C base pairs. Thus, in the wildtype spectrum an overabundance of transitions occurs at a 5'-G-G-G/C3' or 5'-G-T-G/C-3' sequence (where the mutated base is underlined).