BIOLOGICAL ORDER 



Table VI. Mutations Affecting the Life Cycle. 



phage A. This means that they have become insensitive to the 

 repressor produced by phage A. 



It should be recalled here that immunity in a lysogenic bacterium 

 is specific. A bacterium carrying prophage A is immune toward 

 the superinfecting phage A but not toward a nongenetically related 

 phage B. The repressor is specific. And as it controls the expression 

 or activity of the viral nucleic acid, it must be assumed that it has 

 to establish with it some sort of physical relation. The most likely 

 hypothesis is that the specific repressor attaches itself onto a specific 

 spot of the viral chromosome. In the evolution of a temperate 

 bacteriophage toward virulence, two steps have thus been recog- 

 nized. The first is the loss of the ability to produce a repressor. 

 The second is the loss of the sensitivity to the repressor. From a 

 temperate phage, a virulent one has been obtained. It is unable 

 to lysogenize and can only multiply vegetatively, just like the most 

 virulent phages encountered in nature. 



Mutations affecting the vegetative phase. In a normal lysogenic 

 population, any bacterium is potentially able to produce bacterio- 

 phage, and it will produce bacteriophage and lyse if induced. In some 

 strains of lysogenic bacteria, all bacteria lyse after induction, but 

 only one out of 10"^ will produce bacteriophage. Absence of 

 bacteriophage production is due to a genetic defect of the prophage. 

 Defective lysogenics belong to different categories (Table VII). 



[78] 



