The presence of bound acriflavine protects bacteria against the lethal effects of ultraviolet (UV) light, presumably because pyrimidine dimer formation is inhibited. Although acriflavine present in plating medium usually results in reduced viable counts from irradiated bacteria, no enhancement of lethal effects is observed when acriflavine is added to irradiated bacteria left in suspending buffer for 45 min before plating. Acriflavine remaining bound to the deoxyribonucleic acid of irradiated bacteria at the time they are plated likewise does not affect their survival. Protection is precisely dose-modifying unless some killing of bacteria by UV results from induction of prophage, against which bound acriflavine is less protective, or from induction of pyocin, against which there is no protection at all. It is inferred that prophage induction proceeds in part, and pyocin induction wholly, by virtue of effects of UV other than pyrimidine dimerization. The response of Escherichia coli strain B to radiation has been postulated to be attributable in part to induction of a prophage or a lethal protein; but exact dose modification was observed for this strain, to about the same extent, whether or not the irradiated organisms were grown in conditions thought to enhance the expected contribution to killing if such a mechanism were involved. Our results support the hypothesis that the inhibition by acriflavine of dimer formation is attributable to energy transfer mechanisms. They fail to support the hypothesis that shapes of survival curves (in particular the manifestation of “shoulders”) can be attributed to inactivation by radiation of repair enzymes
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