326 BACTERIOPHAGES 



tides were adsorbed to the host cell. This notion seemed to be 

 confirmed when it was found that phage T2 inactivated with 

 ultraviolet light interfered under certain conditions with the 

 multiplication of active phage T2 as well as with the multiplica- 

 tion of the unrelated phage Tl (Luria and Delbriick, 1942). 

 These observations were eventually explained in other ways but 

 for several years the hypothesis of the "key enzyme," essential 

 for phage multiplication, was generally accepted. According 

 to this hypothesis competition between virus particles for a 

 limited amount of this key enzyme resulted in exclusion of all 

 but one of them. The key enzyme hypothesis was discarded 

 and replaced by Delbriick (1945c) with the penetration hy- 

 pothesis already discussed in this chapter. At about the same 

 time clear-cut evidence that more than one phage particle can 

 participate in multiplication within one cell was obtained. 



Delbriick and Bailey (1946), working with the closely related 

 phage strains T2, T4, and T6, demonstrated that any pair of 

 these phages could grow together in the same host cell. Del- 

 briick and Bailey also investigated mixed infection with r"*" and r 

 mutant phages by the single burst technique and found that the 

 majority of mixedly infected cells liberated both kinds of phage 

 particles. Similar observations were reported by Hershey 

 (1946a, b) concerning mixed infection with various mutant 

 phages. The experiments of Delbriick and Bailey (1946) 

 and Hershey (1946a, b) showed that related phages are not 

 subject to mutual exclusion. These experiments were also 

 milestones in the history of phage genetics, but this is related in 

 Chapter XVIII. 



Many examples of mixed growth of closely related phages 

 have been reported since. When the two phages differ by only 

 a few mutational steps there is no exclusion except for the 

 phenomenon of limited participation which presumably occurs 

 also in multiple infection with a single kind of phage (Dulbecco, 

 1949b). With less closely related phages there are likely to be 

 varying degrees of partial exclusion discussed below. With 

 unrelated phages mutual exclusion is likely to be complete. 



