Bacteriophage T4 with essentially complete substitution of 5-bromouracil (BU) for thymine in its DNA was exposed together with control phage to${}^{60}{\rm Co}\text{-}\gamma $ radiation or 16-keV X radiation. As judged by plaque-forming ability, the BU-enhancement ratio for phage lethality was 1.2-1.4 when irradiated in buffered saline, while the enhancement ratio for induction of double-strand breaks in the DNA was about 2.2. In buffered saline, the indirect effects of radiation on phage proteins may account for a portion of the lethality and for the lack of correlation of lethality with double-strand breaks. Moreover, DNA was released into the buffered saline during irradiation; this factor would further confuse any possible correlation between double-strand breaks and lethality under these conditions. The BU-enhancement ratio for phage lethality increased to 1.8 in oxic broth and to 2.3 in anoxic broth plus cysteamine. Under these latter conditions, where indirect effects of radiation should be minimal, most of the lethality in both the control and BU phage could be accounted for by double-strand breaks in the DNA. There was no evidence for any specific interaction of the 16-keV X radiation with BU phage under the experimental conditions used.

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