380 BACTERIOPHAGES 



the host and behaves as a normal cell constituent. Although 

 it carries all the information necessary for phage production, 

 it replicates without fatal intervention in the economy of the 

 bacterium. The presence of the prophage at its specific chro- 

 mosomal site nevertheless endows the bacterium with new 

 properties: the potential ability to produce phage; immunity 

 against superinfection with homologous phages ; and new phys- 

 iological capacities. Thus it is not only the presence, but also 

 the position, of viral material in the cell that characterizes a 

 prophage, and determines the properties of a lysogenic bacterium. 



A lysogenic system is very stable. Only when equilibrium is 

 upset by rare or drastic events can lysogenic bacteria produce 

 phage particles. The development of prophage into phage is 

 a lethal process that brings about the destruction of the host. 

 Under ordinary cultural conditions, this is an infrequent, chance 

 occurrence in the bacterial population. In certain strains, 

 however, prophage development can be initiated in nearly 

 all the cells by exposing them to inducing agents such as ultra- 

 violet light. As a consequence of induction, transition occurs 

 from the prophage to the vegetative state. From then on, the 

 series of events leading to cellular lysis and the liberation of 

 infectious particles appears identical both after infection and 

 induction. 



The lysogenic character is therefore a genetic property of the 

 bacterium that can be acquired by infection with a virus 

 This is the main feature of lysogeny. 



