MUTATION AND PHENOTYPIC VARIATION IN PHAGES 301 



Examples similar to the category last discussed, but unrelated 

 to lysogeny, may exist. Such examples are unknown and in 

 general might be difficult to establish. 



The categories of mutation affecting growth potential sum- 

 marized above are clearly diverse, and probably many more 

 types exist. The utility of considering them together is that in 

 each case a block in one or another step in the sequence of reac- 

 tions essential to phage growth is released by mutation. Such 

 blocks are potentially identifiable in genetic and metabolic 

 terms (Bertani, 1953b). 



c. Mutations Affecting Requirements for Adsorption 



Anderson (1948b) found that a line of T4, for which trypto- 

 phan is a cofactor of adsorption (Chapter X), can mutate to 

 tryptophan independence. Delbriick (1948) isolated similar 

 "biochemical mutants" of T4, including some requiring calcium 

 ion in addition to tryptophan. The requirement for tryptophan 

 (or other cofactors) in T4 ranges all the way from strict depend- 

 ence to complete independence, with intermediate requirements 

 including "temperature sensitive mutants" that are helped by 

 tryptophan at lower temperatures but not at 37 ° C. (Anderson, 

 1948c; Wollman and Stent, 1952). 



The adsorption of Delbriick's (1948) tryptophan-requiring 

 stocks was inhibited by indole, suggesting competitive interaction 

 of tryptophan and indole at a common site of activation. How- 

 ever, Anderson's (1948b) stocks are not inhibited by indole, yet 

 show the same kinetics of activation by tryptophan as indole- 

 sensitive stocks (Anderson, 1948a; Stent and Wollman, 1950). 

 The effects of indole and tryptophan on adsorption of different 

 related phages, illustrated in Table XVII, suggests that these 

 two substances act independently, though perhaps on a common 

 activation mechanism. This mechanism is unknown, but a 

 start has been made toward its identification (Chapter X). 



Burnet and Freeman (1937) isolated from phage H (related to 

 T2) a variant that adsorbed more slowly to its host than did the 

 parental phage. The isolation was accomplished by repeated 



