214 A. GAREN AND L. M. KOZLOFP 



is less temperature-sensitive in T2 than in Tl or T7. (Other differences in 

 this mechanism are discernible in the interaction of T2 and Tl with an ion 

 exchange resin; DNA is released from T2, but not from Tl, after the phage 

 has attached to a resin — see Section V, E). It is noteworthy that, except for 

 the reversibility behavior, the attachment reactions of T2, Tl, and T7 

 respond similarly to a change in temperature; when attachment occurs at 

 instead of at 37°C., the host ceU is not killed and the phage DNA is not 

 injected into the cell. The temperature-sensitive step which is prerequisite for 

 cell killing and DNA injection has not been identified. 



C. Influence of the Medium 



Attachment does not occur in distUled water, probably because both the 

 phage and cell surfaces carry a net negative charge. Attachment usually 

 begins upon addition of an inorganic salt (Puck et al., 1951). The salt require- 

 ment is phage-specific. For example, Tl attaches at optimal velocity in a 

 10-3 ]y[ solution of a divalent cation salt (of Mg++, Ca++, Ba++, or Mn++), or in 

 a 10-2 ]y[ solution of a monovalent cation salt (of Na"*", K"^, Li"*", or NH4"'"), 

 while attachment of T2 is optimal in a 10-^ molar solution of a monovalent 

 cation salt, divalent cations being ineffective. Since any one of a variety of 

 cations are effective with Tl, apparently their only function is to neutralize 

 repulsive electrostatic forces between a phage particle and a cell. With T2, 

 other cation-specific factors must be important, but these have not been 

 identified. 



Attachment generally is measured at pH 7. The effect of changing the pH 

 has been determined for Tl and T2. Both phages attach best in the pH 

 range from 6 to 10, and fail to attach at the pH extremes of 4.8 and 12. This 

 behavior suggests that ionized carboxyl and amino groups are needed for 

 attachment of these phages (Puck and Tolmach, 1954). 



D. Tryptophan Requirement 



Certain strains of the T-even phages cannot attach in a synthetic medium 

 unless first "activated" by exposure to L-tryptophan (Anderson, 1948). The 

 amino acid requirement for activation is remarkably specific; most other 

 amino acids, including D-tryptophan, are ineffective, and a few work only at 

 a concentration considerably greater than is needed with L-tryptophan. 

 Some strains require tryptophan for attachment at 15 but not at 37°C.; 

 others require calcium ions in addition to the amino acid (Delbriick, 1948). 

 Activation appears to require the binding of five molecules of tryptophan to 

 a phage particle, presumably at a critical region of the tail (Stent and 

 Wollman, 1950). The activating effect of tryptophan is rapidly and com- 

 pletely reversed when tryptophan is removed from the medium. Recently it. 



