RADIATION AND VIRUSES 353 



It must be remembered that, in the type of titration employed, only bac- 

 teria which, after receiving phage particles, liberate active phage are 

 measured. Only the.se bacteria are lysed; infected bacteria that fail to 

 liberate active phage die unlysed. 



If the number of bacteria that liberate active phage is determined and 

 the number of residual active phage particles is subtracted from it, the 

 number of bacteria in which inactive phage was reactivated is obtained. 

 This is never higher than the number of cells that receive two or more 

 inactive particles. For small doses of ultraviolet radiations and for high 

 multiplicities of infection, the two become approximately equal. Thus, 

 reactivation is due to intracellular interaction between phage particles 

 which, if adsorbed on separate bacteria, would have registered as inactive. 



The interpretation of the mechanism of "multiplicity reactivation" is 

 at this time obscure. The theory originally proposed for its interpreta- 

 tion (Luria, 1947; Luria and Dulbecco, 1949) is undergoing revisions. 

 When the phenomenon was first recognized, it was quickly discovered 

 that reactivation occurs not only among particles of the same phage 

 but also among particles of two related phages. More especially, it occurs 

 among particles of phages T2, T4, and T6. These exhibit the remarkable 

 phenomenon of genetic recombination, in which mixed infection with two 

 different phages results in the production of "hybrid forms," deriving 

 some of their properties from one phage, some from the other (Delbriick 

 and Bailey, 1946; Hershey and Rotman, 1948, 1949). 



This observation suggested a similarity of mechanism between recom- 

 bination and reactivation, and the hypothesis was formulated that ultra- 

 violet irradiation produced, by discrete hits, a damage localized in discrete 

 gene-like individual "units" in each phage particle and that reactivation 

 resulted from cooperation among the infecting particles. This coopera- 

 tion was supposed to involve the same (unknown) mechanism as that 

 involved in genetic recombination. 



The requirement for reactivation in a given bacterium was then postu- 

 lated to be the possession by the infecting particles as a group of at least 

 one set of undamaged units. This led to the expression, for the maximum 

 frequency of production of active phage, 



00 



I 



x''e 



J., 11 - (1 - e-"-)']' 



where x = average number of inactive particles per bacterium, 

 r = average number of hits per particle, 

 k = an integer number, and 



n = number of the hypothetical units per particle (assumed in first 

 approximation to have equal ultraviolet sensitivity). 



