LYSOGENY 341 



mutations seem to affect the stability of the prophage in the course of bac- 

 terial division. Others affect the very process by which the phage is converted 

 into prophage. The latter mutants, when plated on sensitive bacteria, form 

 clear plaques, in contrast with the turbid centered plaques formed by the 

 wild type. Genetic analysis of these so-called clear {C) mutants in both phage 

 A of *S. tyjihimurium (Levine, 1957) and phage A of £'. coli K12 (Kaiser, 1957) 

 has brought valuable information about the lysogenization process. The C 

 mutants can be subdivided into three classes (Cj, C^, and Cg) according to 

 their phenotype, that is, the aspect of the plaques they form. All of the C 

 mutations appear to be located on a short region of the phage linkage group. 

 Moreover, the C^ mutants are all located in a cluster on a segment of the C 

 region (C^ segment). Likewise, mutants of the Cg and C^ phenotypes are 

 located, also in clusters, on a C^ and a C^ segment, respectively, of the C 

 region. 



Whereas infection of sensitive bacteria with a given C mutant leads to no, 

 or very little, lysogenization, mixed infection with pairs of mutants belonging 

 to different types (C^ -f Cg, C^ + C^, C^ + C^) results in a rate of lysogeniza- 

 tion analogous to that found with the wild type. Lysogenic bacteria isolated 

 after such mixed infections carry either one type of prophage, in the case of 

 Salmonella, or both types, in the case of ^. coli K12. On the contrary, mixed 

 infection with mutants of the same group does not increase the lysogenization 

 frequency. This situation, which is similar to that analyzed by Benzer (1955) 

 in the case of rjj mutants of T2 is equivalent to a case of "pseudo aUelism." 

 Blixed infection with two mutant types simulates a heterozygous diploid in 

 the trans configuration. The application of the phenotype test to pairs of C 

 mutants leads to the division of the region into three functionally separable 

 segment each segment probably controUing a different reaction leading to 

 the prophage state. The nature of these reactions, as well as their sequence, 

 is not yet known. 



VIII. Genetic Determinism of Lysogeny 



A lysogenic bacterium possesses and transmits to its progeny the informa- 

 tion necessary for synthesizing a definite type of phage particles. When the 

 lysogenic system has been established, as a consequence of infection of a 

 sensitive cell by a given type of temperate phage, the phage liberated is 

 identical to the original type. The stabihty of the lysogenic character, once 

 established, impHes an efficient mechanism for the inheritance of this char- 

 acter. Schematically, such a mechanism can be ensured, either by a specific 

 process of repHcation and segregation of the prophage itself as a nuclear 

 structure at each cell division, or by a random process, provided the number 

 of prophages is high enough so that the probability for any daughter cell not 

 to receive at least one copy of it at each division is negligible. 



