BACTERIOPHAGE Q 



On the other hand, there is ample evidence of a deep-reaching 

 aheration of the virus after infection. Doermann, breaking open phage- 

 infected bacteria at different times after infection, found that for several 

 minutes no phage activity could be recovered, and only around the 

 middle of the "latent period" do the first active particles make their 

 appearance (8). A similar conclusion had been reached in my lab- 

 oratory from the study of the effect on phage-infected bacteria of the 

 dye proflavine, which apparently stops phage production, but allows 

 bacteria to lyse and liberate whatever particles were already present 

 when the dye was added (12). Here, too, no active phage is liberated 

 by cells to which acriflavine was added during the first period. Thus, 

 there is an "eclipse" of recoverable phage activity between the dis- 

 appearance of the infecting particles and the appearance of new ones. 

 To the relation between the two I shall return later. 



Another line of evidence indicating that the final particles are not 

 a direct product of reproduction of the infecting particles as such is the 

 occurrence of complex interactions revealed by experiments with mixed 

 infection. Mixed infection of bacteria with pairs of unrelated phages, 

 such as Ti and T2, or Ti and T7, gives "mutual exclusion": only one 

 type of particle is liberated by each bacterium and the infecting particle 

 of the other type is lost, again suggesting an alteration of the infecting 

 particles (7, 4). Phages T2, T4, and T6, however, form a related 

 group and mixed infection of a bacterium with two of them gives rise 

 to a mixed yield. If the two infecting types differ in a character whose 

 alternative forms can manifest themselves in both types — for example, 

 T2r+ and T4r — they give progeny containing, besides the parental 

 types, some new types also, which result from a recombination of the 

 alternative characters r and r+ present in the parental types: T2r+, 

 T2r, T4r+, T4r. This fundamental result, obtained by Delbriick and 

 Bailey (6), was greatly extended by Hershey and Rotman (16, 17) 

 who, studjdng recombination among different mutants of phage T2 

 infecting the same host cells, shov/ed that recombinant types occur 

 with definite specific frequencies for different characters, suggesting a 

 localization of the hereditary properties of bacteriophage in discrete 

 material determinants. We can then, at least formally, interpret recom- 

 bination experiments according to the model of a phage particle com- 

 posed of a number of discrete recombinable genetic units, whose number 

 is probably quite large, of the order of 100 or more. 



Another kind of interaction is "multiplicity reactivation," which 

 consists in the production of active bacteriophage inside bacteria in- 

 fected with two or more particles of some bacteriophages previously 



