368 BACTERIOPHAGES 



The main problem in the study of lysogeny deals therefore with 

 the nature of the prophage-bacterium relationship. The sta- 

 bility of the lysogenic character implies the transmission of 

 prophage to both daughter cells at each bacterial division. 

 Schematically, such a mechanism can be insured in two ways: 

 either a specific process of replication and segregation of the 

 prophage itself as a nuclear structure; or a random process, 

 provided the number of prophages per bacterium is high enough 

 so that the probability for any daughter cell not to receive at 

 least one is negligible. 



All the available evidence supports the former hypothesis. 

 On the one hand, experiments with lysogenic bacteria (Bertani, 

 1953a; Jacob and Wollman, 1953) point to the existence of a 

 small number of prophages per cell, correlated with the number 

 of nuclei (Section 7 below). On the other hand, analysis by 

 crossing between lysogenic and nonlysogenic bacteria (Leder- 

 berg and Lederberg, 1953; Wollman, 1953; Appleyard, 1954a; 

 Fredericq, 1954c; Jacob and Wollman, 1957) as well as by 

 transduction experiments (Lennox, 1955; Jacob, 1955; Morse, 

 Lederberg, and Lederberg, 1956) indicate that the prophage is 

 indeed located at a given site on the bacterial chromosome 

 and constitutes the sole determinant of lysogeny. Moreover, 

 different types of prophage appear to hav^e different locations 

 each one occupying a specific site of the bacterial chromosome 

 (Jacob and Wollman, 1957). In one case, however, the same 

 prophage has been claimed to occupy, with unequal probabili- 

 ties, one of three possible sites (Bertani, 1956). 



A lysogenic bacterium may thus 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 pro- 

 phage is added to, and not substituted for, an homologous seg- 

 ment of the host chromosome. It does not seem to be in- 

 serted into the axis of the bacterial chromosome, but is bound to 

 it in some yet unknown way (.lacob and Wollman, 1957). 



