Effect of UV irradiation on macromolecular synthesis and colony formation in Bacteroides fragilis

Irradiation of Bacteroides fragilis cells with far-UV light resulted in the immediate, rapid and extensive degradation of DNA which continued for 40 to 60 min after irradiation. During the degradation phase, DNA synthesis was decreased but was never totally inhibited. DNA degradation after irradiation was inhibited by chloramphenicol and caffeine. DNA synthesis in irradiated cells was reduced by chloramphenicol but resumed after 100 min at the same exponential rate as in irradiated cells without chloramphenicol. Irradiated cells continued to synthesize DNA for 40 min in the presence of caffeine but after this time DNA synthesis was completely inhibited and never recovered. RNA and protein synthesis were decreased by UV irradiation and the degree of inhibition was proportional to the UV dose. Colony formation was not affected immediately by UV irradiation and continued for a dose-dependent period before inhibition. There was an inverse relationship between UV dose and inhibition of colony formation which occurred sooner in cells irradiated with lower doses of UV light. The characteristics of DNA synthesis in B. fragilis cells after UV irradiation differ from those in wild-type Escherichia coli cells, where DNA synthesis is stopped immediately by UV irradiation, but resemble those in E. coli recA mutant cells where extensive degradation occurs following UV irradiation.