190 A. LWOFF 



the sugars, are synthesized by the enzymatic machinery of the bacterium. It 

 is admitted also that the bacterium provided the Lipmann system, that is to 

 say, the system of enzymes which converts the potential energy of foodstuffs 

 into the high energy bonds which are necessary for syntheses and the produc- 

 tion of a specific pattern. This does not mean at all that bacteriophage is 

 completely devoid of enzymatic activity. When a bacteriophage particle 

 attaches itself to a bacterium, the bacterial wall is altered by an enzyme of 

 the phage tail. Very early during phage development, the bacterial nucleus 

 is destroyed; it is possible that this is due to another phage enzyme. The 

 current view is that during its vegetative phase the bacteriophage diverts a 

 part or the totality of the essential metabolites synthesized by the bacterium 

 and turns them into phage material: nucleic acids and proteins. It acts as a 

 parasite and it is a strict parasite. 



It is important to remember that the bacteriophage particle never is 

 reproduced as such: it neither grows nor divides. An infectious particle is 

 never the direct descendant of another infectious particle. It is produced de 

 novo by the organization of phage material, proteins, and nucleic acids. And 

 it is also essential to know that the vegetative phase of the cycle has its 

 origin in the genetic material of the bacteriophage. 



B. The Proviral Phase and the Lysogenic Bacterium 



The genetic material of the phage does not multiply. It reaches a specific 

 locus or receptor of the bacterial chromosome and attaches itself to it. By 

 virtue of this attachment, the properties of the genetic material of the phage 

 are modified and it is then called a prophage. The prophage behaves as if it 

 were a bacterial gene. It divides, together with the bacterial chromosome and, 

 at bacterial division, is transmitted to each of the daughter bacteria. Bacteria 

 which carry a prophage are called lysogenic. They are endowed with the 

 capacity to produce phage in the absence of infection. The prophage is the 

 material base of this property. It is the structure which carries the informa- 

 tion necessary for the production of phage particles. 



A lysogenic bacterium generally divides without producing bacteriophage 

 particles. The production of infectious entities happens spontaneously, that 

 is to say, without knoMOi cause, with a certain probabihty varying with the 

 systems from 10"^ to 10~^ in the interval of two divisions. In some lysogenic 

 strains, phage production can be induced at will in the totality of the popula- 

 tion by inducing agents, such as UV light. X-rays, organic peroxides, or 

 nitrogen mustard. The current hypothesis is that inducing agents act by 

 modifying the nucleic acid. As a result, the genetic material of the bacterio- 

 phage is detached from the bacterial chromosome, and it ceases to be a 

 prophage. The vegetative phase is initiated, ending in infectious phage 

 particles, and the bacterium is lysed. 



