Escherichia coli cells that have been subjected to gamma irradiation show leakage of components. Those associated with DNA appear in the medium with no lag. The absence of lag is observed even for cells in which the tracer label is far from the polymerase, indicating that the enzyme causing DNA degradation is probably not the polymerase. A temperature-sensitive mutant that does not synthesize DNA at temperatures above 42°C nevertheless shows leakage. The amount, but not the initial rate of leakage, is oxygen-dependent. Leakage after a lag of 40 minutes is observed in many, but not all, strains. This is general, indicating a considerable impairment of the permeability barrier. The strain dependence, temperature dependence, and lack of oxygen sensitivity strongly suggest that the delayed permeability is associated with the induction of a defective prophage. Leakiness in all cells occurs at long times after irradiation. We suggest that this is due to a developing imbalance in enzyme content due to radiation action on transcription.

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