SUMMARY: strain B was sensitized to the action of γ-radiation or ultraviolet (u.v.) radiation by incorporating 5-bromouracil into the DNA of the bacteria. Most sensitization was observed after u.v. irradiation, less after anoxic γ-irradiation and least after aerobic γ-irradiation. Incubation of the bacteria for a few hours after irradiation on a nutrient medium which included chloramphenicol generally resulted in extensive restoration of colony-forming ability whether or not the bacteria contained 5-bromouracil. Only after the aerobic γ-irradiation of bacteria containing no bromoruacil was little restoration obtained, After aerobic or anaerobic γ-irradiation the ‘rescue’ of bacteria containing 5-bromouracil was relatively larger than that observed for bacteria containing no bromouracil. Maximum restoration was obtained after u.v. irradiation and this occurred to about the same extent, whether or not bromouracil had been incorporated into the bacteria. The results suggest that treatment with chloramphenicol decreases the expression of radiation-induced lesions which occur in the bacterial DNA; this may account for the mechanism of action of the inhibitor.


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