146 BACTERIOPHAGES 



tryptophan concentration, so that all phage particles are acti- 

 vated l)y concentrations above 100 ;ug. per ml. independently of 

 temperature. The rate of activation is also markedly tempera- 

 ture-dependent at low tryptophan concentrations but not at 

 high concentrations. These temperature characteristics are 

 consistent with the model proposed. 



The work of Stent and Wollman yielded a satisfactory kinetic 

 model for activation by tryptophan but failed to clarify the 

 nature of the activation process in relation to adsorption. Sato 

 (1956) found that activation could also be brought about by urea 

 under circumstances suggesting that the underlying process 

 resembled protein denaturation. By extension of his idea it 

 might be guessed that all or many phages require a similar acti- 

 vation, differing only with respect to the nature of the requisite 

 cofactors, organic and inorganic. Williams and Fraser (1956) 

 showed that the cementing substance for adsorption is composed 

 of a number of fibrillar structures in the tail of T2, which can be 

 induced to "unwind" under conditions not yet clarified. Pos- 

 sibly activation involves a reversible orientation of these fibrils, 

 and possibly the pH-dependent transformation of T2 described 

 by Taylor, Epstein, and Lauffer (1955) (Chapter IV) is a re- 

 lated phenomenon. 



Jerne (1956) found that one of several antibodies produced in 

 response to immunization with T4 can permanently activate 

 cofactor-requiring strains of this phage. His discussion of this 

 phenomenon is very suggestive in connection with the ideas 

 discussed above. Probably a little more work on requirements 

 for adsorption will elucidate the nature of the activation of T2 

 and related phages by salts and other cofactors. 



A paradoxical fact discovered by Anderson (1945a), that in- 

 fected bacteria form plaques on synthetic agar although the free 

 phage T4 particles do not, was explained by Wollman and Stent 

 (1952). They found that T4, immediately after release from 

 lysed bacteria, is able to adsorb to bacteria without added co- 

 factor. This activated state is transient but deactivation is slow. 

 The natural cofactor responsible for "nascent" activity has not 

 been identified. 



