144 BACTERIOPHAGES 



efficiency of plating. Tryptophan is more effective. Anthranilic 

 acid, indole, and indole-3-propionic acid are inactive. Anderson 

 showed that the role of cofactor is to allow adsorption to occur. 



In a later paper T. F. Anderson (1945b) reported that Bz-3- 

 methyl tryptophan, which inhibits growth of bacteria and phage, 

 can substitute very efficiently for tryptophan as an adsorption 

 cofactor. T. F. Anderson (1946) listed a number of organic sub- 

 stances which had varying degrees of cofactor activity. Substi- 

 tutions or changes in the amino or carboxyl groups of tryptophan 

 destroy all cofactor activity, whereas quite extensive changes are 

 permissible in the remainder of the molecule. Replacement of 

 the indole structure by pyridine, benzene, or thiophene results 

 in a cofactor activity about 1 per cent of that of tryptophan. 

 Delbriick (1948) reported that DL-norleucine had about 0.3 per 

 cent of the activity of L-tryptophan. The remarkable structural 

 specificity of the cofactor is indicated by the fact that L-trypto- 

 phan is the most highly active substance yet tested but o-trypto- 

 phan is inactive. 



By kinetic methods, T. F. Anderson (1948a) showed that 

 cofactor reacts with the phage particles rather than with the host 

 bacteria. This process is termed "activation." Anderson esti- 

 mated that about six molecules of tryptophan are involved in the 

 activation of each phage particle. There is a rather high tem- 

 perature coefficient for activation with a temperatu»'e optimum 

 at about 35 ° G. When activated T4 is diluted in a tryptophan- 

 free medium, the phage very rapidly become "deactivated." It 

 is evident that phage T4 reacts in a reversible manner with 

 tryptophan to form an activated complex capable of adsorbing 

 to the host cell. 



A small portion of the particles of phage T4 in certain stocks 

 are able to form plaques when plated on tryptophan-free agar. 

 When these plaques are picked and subcultured they give rise to 

 strains which can adsorb in the absence of tryptophan. Thus the 

 ability to adsorb lo the host cell in the absence of cofactors is a 

 genetically determined characteristic of ihc j^hage (T. F. Ander- 

 son, 1948b). 



