MUTATION AND PHENOTYPIC VARIATION IN PHAGES 293 



issuing from different cells of the modifying host seem to contain 

 identical proportions of particles capable of growing in (excep- 

 tional cells of) the refractory host, in contrast to the expectations 

 for mutational changes in the properties of the phage (Luria, 

 1953b; Ralston and Krueger, 1954). However, it does not 

 follow that the growth potential of all the particles of the modi- 

 fied phage is identical (Luria, 1953b). 



3. In general, all of the phage particles issuing from a given 

 cell of the modifying host exhibit the altered host range. In 

 most instances, too, all the cells of the modifying host produce 

 the same modification of the phage, if they produce phage at all. 

 In one instance this is not true. One strain of B/4 contains cells 

 of which some yield "normal" T2 and soine restricted T2, de- 

 pending on the age of the culture at the time of infection (Luria, 

 1953b). In other instances, the modifying tendency of the host 

 is determined, in part, by carried prophages unrelated to the 

 phage undergoing modification (Chapter XXI). 



The phenomenon of transduction (Chapter XIX) also fur- 

 nishes examples of nonheritable variations of phage particles 

 controlled by the host. These variations differ from the ex- 

 amples cited above in several respects. In most cases only a 

 minority of the particles is affected. The alteration may or 

 may not aff'ect the growth potential of the phage. The mecha- 

 nism is presumed to be quite different from that responsible for 

 other examples of host-controlled variation. The only reason 

 for mentioning transduction here is the possibility that it may 

 possess unknown features coinmon to all host-induced modifica- 

 tions. 



In summary, nonheritable modifications of host range are 

 produced with very high efficiency when phages multiply in 

 certain hosts. Since these do not in general affect the ability of 

 the phage particles to complete the initial steps of infection, the 

 alteration is probably localized in soine of the injected inaterials 

 (Luria, 1953b). The next question is unanswered: are the 

 modifications localized in DNA or in some of the minor compo- 

 nents of the phage particles? 



