DEGRADATION OF THE DNA OF ESCHERICHIA COLI K12 REC‐ (JC1569b) AFTER IRRADIATION WITH ULTRAVIOLET LIGHT*

Degradation of the DNA of a rec‐ mutant of Escherichia coli K12 (JC1569 b) induced by u.v. light was investigated. The rate of degradation was much larger by growing bacteria than by stationary cells. When growing bacteria were starved for amino acids, their DNA became resistant to irradiation. The mode of u.v.‐induced degradation was investigated by comparing the time course of release from the acid‐insoluble fraction of the label for two growing cultures; the one was pulse‐labeled with 3H‐thymidine and the other was pulse‐labeled and chased thereafter for 12 min. It was found that the label incorporated into the former culture begins to be lost from the acid‐insoluble fraction prior to the loss of the label incorporated into the latter culture. It was concluded that breakdown of the replicating point precedes degradation of the bulk of the DNA. This result suggested that the replicating point is a sensitive site to irradiation and the u.v.‐induced degradation of DNA seemed to be influenced by the state of chromosome at the time of irradiation. Experiments of centrifugation of lysed spheroplasts of bacteria uniformly labeled with 3H‐thymidine in alkaline sucrose demonstrated that DNA of low molecular weight appeared after irradiation with only 5 ergs/ mm2, and that the molecular weight could not be restored by post‐irradiation incubation. Considering these results, an hypothesis is proposed concerning the initiation of induced degradation of the DNA of the rec‐ mutant.