340 F. JACOB AND E. L. WOLLMAN 



Another effect on lysogenization has been observed by Boyd (1952) in 

 Salmonella typhimurium infected wath phage A. When the multiphcity of 

 infection is small (one or less), most of the infected bacteria lyse and produce 

 phage. When the multiplicity of infection is gradually increased, the propor- 

 tion of infected cells that become lysogenic increases too. When the cells are 

 infected with an average of 10 particles per bacterium, the fraction of lyso- 

 genics reaches almost 100 %. Thus, the infecting particles can cooperate in 

 estabhshing lysogeny. In this system the response to infection is also depen- 

 dent on cultural conditions (Lwoff e^ al., 1954). The lysogenization frequency 

 for a given multiplicity of infection can be influenced by such treatments as 

 starvation of the cells or addition of various metabolites and antimetabolites 

 to the cultures. The factors which may shift the responses toward lysogeniza- 

 tion or phage production are efiicient only during the first 6 or 8 minutes 

 following infection. Only at the end of this period does an irreversible reaction, 

 leading to one or another type of response, take place. Cooperation between 

 infecting particles is also effective during the first minutes following infection 

 (Lieb, 1957). When bacteria are first infected with small multiplicities of 

 phage A, superinfection with high multiplicities of the same phage or its 

 mutants will increase the lysogenic responses only during the first few 

 minutes, but not later. 



In the case of E. coli K12 and phage A, analyzed by Lieb (1953), the factors 

 influencing the decision between lysis or no lysis appear to be stiU more 

 complex. A rapid and apparently irreversible modification seems to be under- 

 gone by the infecting particle, leading towards either the vegetative or the 

 prophage state. Among the ceUs that wiU not lyse, some give rise to clones 

 which consist exclusively of lysogenic ceUs, others to clones of sensitive cells; 

 the remaining clones are mixtures of sensitive and lysogenic cells. The pro- 

 portion of clones that contain only lysogenic ceUs and also the proportion of 

 lysogenics in the mixed clones can be increased by increasing the multi- 

 plicity of infection or by decreasing the temperature of incubation for one 

 hour after infection. This suggests that those particles which have not 

 undergone the transition to the vegetative state do not divide before becom- 

 ing a prophage and at each bacterial division are randomly distributed 

 among the daughter cells. During this period, the phage material is very 

 sensitive to temperature: it can be inactivated by incubation at 44°C. Only 

 after the end of the first hour does the phage material behave as a prophage: 

 it divides in harmony with the cell and exhibits the same temperature 

 sensitivity as the host. 



B. Genetics of Lysogenization 



In temperate phages, various mutations occur which alter or even suppress 

 the capacity for lysogenization exhibited by the wild type. Some of these 



