Mutagenic specificity of N-methyl-N'-nitro-N-nitrosoguanidine in the tonB gene on the chromosome of Escherichia coli recA(+) and recA(-) cells

DNA base sequence changes induced by N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) mutagenesis has been determined for the endogenous tonB gene of Escherichia coli recA(+) strain and its isogenic recA56 strain. In the recA(+) strain, base substitutions accounted for 48 mutations among 54 MNNG-induced independent mutations analyzed and consisted of 45 G:C to A:T transitions, two A:T to G:C transitions and one A:T to C:G transversion. In the recA56 strain, 67% (34/51) were base substitutions among which G:C to A:T transition (28/51) predominated, followed by three A:T to T:A transversions, two G:C to T:A transversions, and one A:T to G:C transition. These mutagenic specificities were consistent with the mispairing predicted by the methylation of the O-6 position of guanine. In both strains the G:C to A:T mutations were found at guanine residues preceded by either guanine or thymine on the non-transcribed strand of the target gene. (C) 1996 Academic Press, Inc.