N-Ethyl-N-nitrosourea-induced mutations occurring within a 180-base pair target in the lacI gene of Escherichia coli were characterized by DNA sequencing. In total, 109 mutations were characterized in a wild-type background and 100 in an excision-repair-deficient (UvrB-) background. The majority of mutations induced in the two backgrounds (77 and 85%, respectively) were G:C = >A:T transitions, presumably resulting from miscoding O6-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 analysis 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 O6-ethylguanine and O4-ethylthymine lesions which are flanked on both sides by G:C base pairs. Thus, in the wild-type spectrum an overabundance of transitions occurs at a 5′-G-G-G/C-3′ or 5′-G-T-G/C-3′ sequence (where the mutated base is underlined).

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This research was supported by a strategic grant (G1598) from the Natural Science and Engineering Research Council of Canada.

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