LYSOGENY 345 



lines of evidence suggest that the prophage is indeed a DNA structure. First 

 of all, the abilities of temperate phages either to multiply or to lysogenize 

 are equally sensitive to UV inactivation or P^^ disintegration both before 

 and after infection of the sensitive host (Garen and Zinder, 1955; Stent and 

 Fuerst, 1956). Like vegetative phage, the prophage appears to evolve, upon 

 lysogenization, from the DNA of the infective particle. Moreover, lysogenic 

 bacteria do not contain any recognizable antigen of the homologous phage 

 (Miller and Goebel, 1954). Finally, the prophage has recently been shown to 

 exhibit the same sensitivity to P^- decay as the homologous infectious par- 

 ticle (Stent et al., 1957). This result would indicate that the prophage con- 

 tains the same amount of phosphorus atoms and, probably, of DNA as the 

 genetic material of the free phage. 



Another important point which still remains unclear is what makes the 

 basic difference between the prophage, which replicates together with the 

 nuclear structures of the bacterium, and the vegetative phage, whose multi- 

 pHcation brings about the destruction of the host. Such a difference in the 

 behavior of the two types of structures can hardly be ascribed to a differ- 

 ence in their nature but rather in their activity. In a lysogenic bacterium, the 

 prophage (probably DNA) replicates, but apparently does not synthesize 

 any phage protein. During the vegetative phase, not only does the genetic 

 material replicate, but also phage proteins are produced. This is the only 

 quaUtative difference so far known between the lysogenic and the vegative 

 phase of the life cycle of a phage. 



Obviously, the chromosomal location of the prophage explains most of the 

 known facts relevant to lysogeny. A lysogenic bacterium may be visualized 

 as possessing the genetic material of a phage located at a specific site of the 

 bacterial chromosome. The two integrated structures replicate as a whole. 

 Different lines of evidence suggest that the prophage is added to, and not 

 substituted for, a homologous segment of the host chromosome. It does not 

 seem to be inserted in the axis of the bacterial chromosome, but bound to it 

 in some yet unknown way (Jacob and WoUman, 1957). 



The presence of this added genetic material provides the host cell with 

 some specific properties, such as ability to make phage and immunity against 

 homologous particles. When the prophage is inducible, it also determines a 

 high sensitivity to radiations and inducing compounds. In some cases, this 

 new structure may also modify some functional property of the host, such as 

 toxin or antigen syntheses, which at first sight appear to be imrelated to 

 phages. 



IX. Temperate Phages as Genetic Vectors 



The genetic analysis of lysogeny has unraveled the existence, in lysogenic 

 bacteria, of an intimate connection between the genetic materials of a 



