VIRAL FUNCTIONS: ORDER AND DISORDER 



A lysogenic bacterium was defined as a bacterium that perpetuates 

 the power to produce bacteriophage in the absence of infection. 

 From time to time, however, the lysogenic bacterium does produce 

 phage, and we have to know why viral functions are sometimes 

 expressed and sometimes not. The study of lysogeny should provide 

 the answer. The simplest hypothesis is that viral functions are not 

 expressed in a lysogenic bacterium because they are blocked by a 

 repressor. 



ImiMumty. a lysogenic bacterium perpetuates the potentiality 

 to produce phage. But thanks to the presence of the prophage the 

 lysogenic bacterium exhibits another property, the so-called im- 

 munity. Let us assume that our bacterium has been lysogenized by 

 a temperate bacteriophage A and therefore perpetuates a prophage A. 

 The superinfection by the homologous bacteriophage A does not 

 result in a vegetative phase. The genetic material of the infecting 

 bacteriophage A is injected into the lysogenic bacterium, but it 

 behaves as an inert particle. Viral functions are not expressed. Phage 

 proteins are not produced. Moreover, the genetic material of phage A 

 does not multiply and is diluted out in the course of bacterial 

 multiplication (Figure 29). This immunity is specific. Other types 

 of bacteriophage, let us say B, not genetically related to bacterio- 

 phage A can multiply vegetatively, just as if prophage A were not 

 present. Thus, something must be present in the cytoplasm of the 

 lysogenic bacterium carr\ing prophage A which prevents specifi- 

 cally the vegetative multiplication of the genetic material of phage A. 

 Here again a specific repressor of viral functions can be visualized. 



Induction. The probability that a lysogenic population will pro- 

 duce infectious particles varies with the strains from 10 ~- to 10~" 

 per bacterium per generation. This is the so-called spontaneous 

 phage production, spontaneous because \\e do not know its cause. 

 In some lysogenic strains, the frequency of spontaneous production 

 cannot be modified. In other strains, the production of bacteriophage 

 can be induced at will in practically all of the population. These 

 strains are called inducible. 



Inducible lysogenic bacteria are irradiated with a suitable dose 

 of ultraviolet light. The bacteria continue to grow for about 45 

 minutes. Then within 10 minutes, the bacterial population disappears. 

 All the bacteria have been lysed, and each one has liberated some 



[71] 



