Evidence for two modes of DNA degradation in Escherichia coli following ultraviolet irradiation

UV-induced DNA degradation is found in wild-type E. coli strains and is greatly reduced by photoreactivation. UV-induced DNA degradation is not found in an excision repair-deficient strain (uvr A) unless incubated post-UV in the presence of chloramphenicol. Chloramphenicol-enhancible degradation is significant and is not reduced by photoreactivation. Greater DNA degradation is found in cells irradiated frozen at -790C than in cells irradiated at ambient temperature (210C). When the cells are irradiated at -790C the amount of degradation is very slightly decreased by photoreactivation. Chloramphenicol-enhancible DNA degradation does not occur in a lexA strain. These data suggest two modes of UV-induced DNA degradation, one responding to pyrimidine dimers and one responding to lesions other than dimers, with chloramphenicol-enhancible degradation corresponding to the second mode