Photoreactivation of Escherichia coli reverses umuC induction by UV light.

UV mutagenesis in Escherichia coli depends on the presence of a premutagenic lesion in DNA and on the induction of the umuCD gene product as part of the SOS response. Using operon fusions between the E. coli lacZ gene and the SOS genes umuC, uvrB, and dinD, we have affirmed the expected role of the cyclobutane pyrimidine dimer in inducing SOS gene transcription. In addition, we found that photoreactivation of cyclobutane pyrimidine dimers reversed umuC induction to the same extent as it reversed mutagenesis. Therefore, the photoreactivability of UV mutagenesis does not itself identify the cyclobutane pyrimidine dimer as the UV mutagenic lesion in E. coli.