Phage induction by ultraviolet radiation in strains of Escherichia coli possessing and lacking dark repair capacity

Abstract

Prophage induction by ultraviolet light (UV) in strains of Escherichia coli possessing and lacking dark repair capacity was investigated. E. coli B and its UV-sensitive mutant B s-1 were lysogenized by temperate phage ∅80. Relative prophage induction by UV in the two strains paralleled relative UV killing, suggesting that prophage induction in a UV-resistant strain is susceptible to dark repair. Photoreactivation of induction was observed in B s-1(80), suggesting that the initial damage for UV induction of prophage in this very sensitive strain is due to pyrimidine dimers. No recovery of induction, however, was observed in B(80) when the UV-irradiated cells were held in phosphate buffer at 37° although there was recovery of cell survival (liquid holding recovery). The frequency of spontaneous induction was very much greater in B s-1(80) than in other strains and is one of the highest yet observed in bacteria. Although increased sensitivity to UV killing in the logarithmic phase is usually seen in lysogenic strains, B s-1(80) was not more sensitive to UV killing than logarithmic phase B s-1. Cell division after UV irradiation and before lysis, permitting viable cells among the progeny, could account for the lack of extra sensitivity in B s-1(80). This phenomenon actually was observed microscopically, although the mechanism is not understood.