The presence of an inducible inhibitor of postirradiation DNA degradation has been shown in several strains of E. coli. The time course of induction has been followed by adding rifampin at various times after induction by ultraviolet light. The dose dependence of induction has been observed for both uv and ionizing radiation and parallels that for the induction of phage λ. Using rifampin, the presence of some concomitant DNA synthesis in a cell which is degrading DNA has been shown. The experiments suggest that DNA synthesized after irradiation is not degraded. The induction of the inhibitor is an all-or-nothing process, and a culture of cells which has been induced and then irradiated shows no degradation in most cells and full degradation in the remainder. Cells with the inducible inhibitor show a variation in the survival of colony-forming ability in rich versus lean medium; cells lacking the inhibitor do not show the effect. It is suggested that this is related to the rescue of one or more genomes by the action of the inhibitor.

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