Thymine14 C labelled E. coli B / r were irradiated with electrons in saline M / 15) in the presence of, or after treatment with, solutions of various compounds which either enhanced or dininished the effects of radiation on cell survival. Loss of radioactivity from the acid-precipitable DNA was used as a measure of DNA breakdown. Thiourea, glycerol, dimethyl sulfoxide, N-ethyl maleimide, indane trione, and acriflavine were used at the optimum concentrations for protection or sensitization (cell survival). Two of the protective agents, thiourea (0.3 M) and dimethyl sulfoxide (1.0 M), almost completely inhibited post-irradiation breakdown when bacteria were irradiated in their presence, while the third, glycerol (1.0 M), had little or no effect. Of the three sensitizing agents, indane trione (<tex-math>$8\times 10^{-4}\ M$</tex-math>) and acriflavine (<tex-math>$2\times 10^{-5}\ M$</tex-math>) completely inhibited degradation, but N-ethyl maleimide (<tex-math>$5\times 10^{-4}\ M$</tex-math>) had only a slight inhibitory effect. When bacteria were exposed to these compounds before irradiation and then washed, both indane trione and acriflavine still inhibited DNA degradation. None of these six compounds affected cell survival after pre-irradiation washing. It was concluded that for this strain of bacteria, DNA degradation could be inhibited while the colony forming ability remained the same or was greater or less than in cells in which DNA degradation was not inhibited.

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