VIRAL FUNCTIONS: ORDER AND DISORDER 



Infection and lysogenization. A nonlvsogenic bacterium in- 

 fected by a temperate phage may either produce infectious particles 

 and be Ivsed, or become Ivsogenic. The outcome of the infec- 

 tion depends on the genetic constitution of the temperate bacterio- 

 phage. But with a given temperate bacteriophage, the behavior of 

 the bacterium/phage system, and the fate of the bacterium, are 

 controlled by extrinsic factors. 



The fraction of the infected bacteria that will be lysogenized 

 varies effectively from less than \^c to more than 99%. A high 

 temperature (40°C.) favors the establishment of the lytic cycle, 

 whereas a low temperature (20°C.) acts in favor of lysogenization 

 (Figure 31). 



The low or high temperature acts only during the first 7 minutes 

 after infection. After the seventh minute the decision is made and 

 cannot be modified. Something irreversible has happened. Thus, the 

 experimental data point toward the conclusion that the response of 

 the infected bacterium depends on a primary determining event. 

 What is this event? 



It was found that antibiotics which block protein synthesis, such 

 as chloramphenicol, favor the lysogenic response. The hypothesis 

 is that the first event can be the synthesis either of a protein or of 

 something else, namely, a repressor. If a protein is produced first, 

 the vegetative phase is initiated, and the process is irreversible from 

 then on. More proteins are synthesized, the viral DNA is duplicated, 

 and finally infectious particles are produced. If a repressor is pro- 

 duced first, the expression of the viral potentialities is blocked. 

 Proteins cannot be synthesized, and the autonomous cytoplasmic 

 viral DNA is unable to duplicate itself. But, thanks to the correspond- 

 ence of structure with a given chromosomal locus, the viral DNA 

 attaches itself to this receptor site. It is now a prophage. The 

 prophage multiplies; later on, we shall try to understand why it 

 does. It continues to produce the repressor, and the bacterium is 

 now lysogenic. The repressor hypothesis needs of course to be 

 reinforced, and it will be reinforced by the study of viral mutants. 



Mutations affecting inducibility. The genetic material of a 

 bacteriophage, like any other genetic material, may undergo muta- 

 tions. The wild-type A ind+ is inducible with ultraviolet light. A 

 mutant ind^ was discovered which had lost its original inducibility 



[75] 



