LYSOGENY 373 



2. Induction requires exposure of lysogenic bacteria to the 

 action of inducing agents. Several inducing agents are known, 

 including ultraviolet light (LwofT, Siminovitch, and Kjeldgaard, 

 1950), ionizing radiations (Latarjet, 1951), nitrogen mustard, 

 ethyleneimines, epoxides, and organic peroxides (Lwoff, 1953; 

 Jacob, 1954a). All of these agents are also known to exhibit 

 mutagenic or carcinogenic activity. 



Ultraviolet light allows a simple and accurate determination 

 of dose-effect curves for induction of phage development. As 

 a function of dose of ultraviolet light, the fraction of bacteria 

 producing phage first increases, then reaches a maximum which 

 may be greater than 95 per cent, and finally decreases according 

 to a rate which is generally controlled by the bacterial "ca- 

 pacity" to reproduce the homologous phage as measured by 

 irradiation before infection (Jacob, 1954a). 



3. Finally, the bacterial response to inducing agents re- 

 mains under the control of the physiological condition of the cul- 

 tures (Lwoff, 1951). For example, the "aptitude" of inducible 

 lysogenic bacteria to produce phage after irradiation is greatly 

 reduced if they have previously been starved (Jacob, 1952a; 

 Borek, 1952). 



Although little is known about the process of induction, sev- 

 eral lines of evidence indicate that the primary lesion initiated 

 by inducing agents does not affect the prophage itself, but the 

 bacterial host, the conversion of the prophage to the vegetative 

 state being only a secondary event. On the one hand, careful 

 analysis of induction by X-rays indicates that the target size is 

 much too large to be the prophage itself, but is of the same order 

 of magnitude as the whole bacterial nucleus (Marcovich, 1956). 

 On the other hand, analysis of lysogenic bacteria carrying two 

 different, but related, prophages which can simultaneously de- 

 velop in the same bacterium points to the existence of a correla- 

 tion in the production of both types of phage. Such a correla- 

 tion is incompatible with the hypothesis of a change in the 

 prophage itself as the primary event of induction (Jacob, 1952b). 

 Prophage development must therefore result from some bacterial 



