Correlation between transcription and C to T mutations in the non-transcribed DNA strand

We recently showed that transcription can promote C to T mutations in the non-transcribed strand in E. coli. To study the relationship between the level of transcription and mutant frequency, an inactive allele of the kanamycin-resistance gene was expressed under the control of a hybrid promoter consisting of an UP element and the tac promoter. When this promoter is induced, the frequency of C to T mutations in the non-transcribed strand increases in rough proportion to the amount of mRNA. At the highest level of transcription at which cell growth is not affected, there is about a 10-fold increase in the frequency of mutations. This result is consistent with the hypothesis that transcription forces the non-transcribed strand to be in a single-stranded state and that this results in frequent C to T mutations.