LYSOGENY 337 



however, mutants have been found which are able to multiply as efficiently 

 on lysogenic as on nonlysogenic derivatives of the same strain (Bertani, 

 1953a; Lederberg and Lederberg, 1953; Jacob and Wollman, 1953). In view 

 of their ability to overcome the immunity conferred on lysogenic bacteria by 

 a homologous prophage, these mutants may be called virulent mutants 

 (Jacob and Wollman, 1953, 1954). 



In certain systems, the frequency of phage mutations can be enormously 

 increased by irradiating both phage particles and sensitive bacteria before 

 infection, as shown by Weigle (1953) with E. coli K12 and phage A. If irra- 

 diated cells are infected with irradiated phages, a large fraction of the 

 "inactive" particles are reactivated, among which up to 5 or 10 % produce 

 a pure progeny of mutant type. The same increase in the mutation frequency 

 results also from induction of lysogenic E. coli K12(A) with rather high doses 

 of UV light. No effect is observed after irradiation of the phage alone. A 

 certain increase in the frequency of some mutations has been observed after 

 infection of irradiated bacteria with nonirradiated phage (Jacob, 1954; 

 Hershey et al., 1954). When mutants of temperate phages differ by two or 

 more characters, genetic recombination between these characters may be 

 observed either after mixed infection of sensitive cells or after infection of 

 UV-induced lysogenic bacteria carrying a prophage of one type with particles 

 of the other type. The main features are essentially similar to those which have 

 been observed with virulent phages and the theory of Visconti and Delbriick 

 (1953) can be applied to temperate phages. The average number of rounds 

 of matmg is rather low, 0-5 or less (Murphy, 1953; Wollman and Jacob, 1954; 

 Kaiser, 1955). When genetic recombination has been carefully analyzed, 

 temperate phage have been found to exhibit a single linkage group, on 

 which all the known mutations can be mapped. Finally, an increase in 

 recombination frequency between any pair of markers has been observed by 

 exposing to low doses of UV light either the phage particles before infection 

 or the phage-bacterium complex after infection (Jacob and Wollman, 1955). 



Genetic recombination between temperate phages and their mutants, 

 whose lysogenizing power is either decreased or lost, allows a genetic analysis 

 of the process of lysogenization (see Section VII, B). 



D. Prophage Mutations : Defective Lysogenic Bacteria 



Many of the mutations which aifect temperate phages have also been 

 found to occur in the prophage state. Other prophage mutations exist which 

 are of special interest because they prevent the formation of phage particles, 

 and may thus be detected and analyzed only in the prophage state. Strains 

 carrying such mutant prophages are called defective lysogenic strains. 



Defective lysogenic strains of various species have been isolated, either 

 VOL. 11—22 



