APPENDIX 503 



been found for T2, T3, T4, and T7. None have been reported 

 for T5 and T6. Host range mutants in the T group of coli- 

 phages have been discussed by Luria (1945a, 1946) and by Her- 

 shey (1946b). 



Phage stocks, when used for certain purposes, should be rela- 

 tively free from host range mutants. For instance, a high pro- 

 portion of Tl/z in a stock of Tl phage will interfere with the iso- 

 lation of the B/l,tr mutant, since this mutant is lysed by Tlh. 

 Also, a high incidence of host range mutants of T2, e.g., in a 

 stock of T2, may interfere with use of the indicator strain B/2 in 

 mixed infection experiments, because a T2h particle will form a 

 plaque on B/2 and may be interpreted as T4 or T6. The pro- 

 portion of host range mutants in a stock may be found by assaying 

 appropriate dilutions of the stock on B and on the resistant mu- 

 tant of B. It is commonly found that the efficiency of plating of 

 the host range mutant on the phage resistant bacterial mutant is 

 low when compared with assays on strain B. This must be con- 

 sidered when the host range mutants of the phages themselves 

 are to be studied, in mixed infection experiments, for instance. 



2. Adsorption Co/actor Mutations 



T. F. Anderson (1945a) discovered that certain stocks of col i- 

 phages T4 and T6 required presence in the medium of trypto- 

 phan in order that adsorption to the host cell might take place. 

 Further investigation of this phenomenon disclosed that the 

 tryptophan reacted in a reversible manner with the phage parti- 

 cles to form an "activated complex" which then became capable 

 of adsorption to the host cell. At concentrations of tryptophan 

 of 0.1 jug./ml. or less there was no detectable activation, whereas 

 at 2 jug. /ml., activation of the phage was maximal, all phage par- 

 ticles present being capable of adsorption. Anderson also was 

 able to demonstrate that his stocks of T4 phage were inhomogene- 

 ous with respect to adsorption cofactor requirement (T. F. An- 

 derson, 1 948b) . If high concentrations of T4 phage were plated 

 on agar made with a chemically defined medium free of trypto- 

 phan a few plaques would be produced. These plaques gave 



