330 F. JACOB AND E. L. WOLLMAN 



culture controls aptitude, that is, the ability for bacteria to respond to irra- 

 diation by phage production (Lwoff, 1951). Disturbances in bacterial meta- 

 bohsm may affect aptitude markedly, as shown by the effects of carbon or 

 nitrogen starvation before irradiation. In starved populations, both the pro- 

 portion of bacteria able to produce phage and the sensitivity to UV hght of 

 these bacteria are strongly reduced (Jacob, 1952a). Amino acid starvation 

 of amino acid-deficient strains produces the same effect (Borek, 1952). Thus, 

 aptitude does not behave as an absolute property of the system, but under- 

 goes variations controlled by the nutritional conditions of the system. 



After exposure to radiations, environmental factors may still affect the 

 bacterial response (Lwoff, 1951). Changes in the composition of the medium, 

 or in the cationic balance (Huybers, 1953) after irradiation may thus prevent 

 the production of phage in certain systems. 



Like many other effects of UV irradiation, induction may be reversed by 

 exposing irradiated bacteria to visible light (Jacob and WoUman, 1953). 

 Photorestoration of the effects of induction does not take place after exposure 

 to such agents as X-rays or nitrogen mustard. 



Reversion of the effects of irradiation with UV Hght is only effective 

 during the first quarter of the latent period. The bacteria thus recovered as 

 colony formers stiU remain lysogenic. 



D. Mechanism of Induction 



Little is known about the mechanism by which phage development is 

 initiated after exposure of lysogenic bacteria to an inducing agent. The 

 question may be raised as to whether the primary effect is on the bacterial 

 or on the prophage component of the lysogenic system. Two different kinds 

 of evidence support the former hypothesis. One comes from the estimation 

 of the size of the induction target by means of X-rays: induction appears to 

 result from a single ionization and the size of the target found is too large to 

 be the prophage itself. It appears to be analogous to the size of the whole 

 bacterial nuclear system (Marcovich, 1956a). The other evidence comes from 

 the analysis of spontaneous and induced phage production by polylysogenic 

 bacteria carrying two different, but related inducible prophages: the correla- 

 tion found in the productions of both types of phages (Jacob, 1952c) is incom- 

 patible with the hypothesis of a change of the prophage itself as the primary 

 event of phage production. Development of the prophage appears therefore 

 as a secondary effect, the primary event consisting of some alteration of the 

 bacterium. The very nature of the inducing agents suggests that it could be 

 an alteration in the nucleic acid economy of the host. It appears that in 

 lysogenic bacteria the potentiahties of a prophage cannot be expressed as 

 long as the bacterial nuclear apparatus maintains control of the ceU. Only 

 when some disturbance occurs, either spontaneously or after the action of 



