ADSORPTION OF PHAGE TO HOST CELL 145 



Delbriick (1948) isolated additional cofactor-requiring vari- 

 ants of T4. One of these requires calcium ion in addition to 

 tryptophan for adsorption to take place. The calcium require- 

 ment can not be met by magnesium ion. A second strain ad- 

 sorbs well in the presence of tryptophan without either Mg or Ca 

 ions. The adsorption of both tryptophan-requiring strains is 

 inhibited by the presence of indole in the medium. Anderson's 

 tryptophan-requiring stock, however, is indifferent to indole. 

 The sensitivity to indole also varies among different lines of T2, 

 none of which requires tryptophan. 



The reversible activation of phage T4 by tryptophan and the 

 adsorption of the activated complex to the host cell make an 

 interesting kinetic problem which was analyzed in a series of 

 beautiful papers by Wollman and Stent (1950) and Stent and 

 Wollman (1950, 1951), confirming and extending the previous 

 work ofT. F. Anderson. The deactivation of the T4-tryptophan 

 complex is a first-order reaction. The rate of activation by 

 tryptophan is proportional to the fifth power of the tryptophan 

 concentration at low concentrations, but is independent of 

 tryptophan concentration at high concentrations. This indi- 

 cates that five tryptophan molecules must react at a single site 

 to form an activated phage particle. The limiting rate of activa- 

 tion at high concentrations is attributed to a step involving re- 

 arrangement of the five adsorbed tryptophan molecules into a 

 specific and relatively stable configuration. 



The rate of deactivation is markedly sensitive to tryptophan 

 concentrations below the level at which measurable activation 

 occurs. This is explained by the assumption that deactivation 

 involves loss of a single tryptophan molecule from the organized 

 site leaving the remaining four molecules in a specific configura- 

 tion. The active configuration can then be restored by the addi- 

 tion of a single molecule at a rate much faster than that charac- 

 teristic of the primary activation. 



The fraction of the phage population activated by a given con- 

 centration of tryptophan decreases very rapidly with decreasing 

 temperature. This effect can be overcome by increasing the 



