SUMMARY: Members of the Q1 (A) group of lysogenic strains of have a common bacterial component and differ only in their prophage content. All are inducible by ultraviolet (u.v.) radiation but, in the cultural conditions which were standard in the experiments here described, never more than 40% of the bacteria were induced, and this only with exposures which killed approximately 75% of the host strain, Q1. The latent period between irradiation and phage liberation was approximately 45 min. The number of particles per burst varied widely. The most striking difference in the reaction to irradiation of cultures of the different strains was that, without significant variation in the amount of free phage produced, some showed clearing while others did not. Clearing was not directly related to the percentage of bacteria induced, nor was it attributable to the production of a lysin by some strains and not by others. Clearing occurred in strains in which induction took place in bacteria which would otherwise have survived irradiation, and would have enlarged into filamentous forms. In strains in which no clearing occurred, induction took place in bacteria which had received a lethal dose of u.v. radiation, and not in the more lightly irradiated bacteria which were therefore left free to enlarge and so mask the lysis of the induced bacteria. Thus there is evidence that the induction of these closely related prophages is ‘triggered off’ by different stimuli directly related to the degree of damage to the host bacterium caused by irradiation.


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