THE INITIATION OF BACTERIOPHAGE INFECTION 215 



was discovered that the trj'ptophan-activated state can be stabihzed against 

 deactivation in the absence of tryptophan by a component of antiphage 

 serum which does not have phage-neutrahzing activity (Jerne, 1956). 



Another effect of tryptophan on the strains that require activation is to 

 render the tail of the phage more sensitive to irreversible structural changes 

 inducible by heating (Cheng, 1956) or by exposure to zinc cyanide com- 

 plexes (Kozloff et al., 1957), which implies that tryptophan may exert its 

 activating effect by opening or weakening some of the bonds in the tail 

 proteins. This interpretation gains support from the finding that the protein- 

 denaturing agent, urea, also activates attachment of these strains although, 

 unlike tryptophan, it causes additional changes which are lethal (Sato, 1956). 

 The role of activation in the attachment reaction may be the unwinding of tail 

 fibers to some extent in order to expose them to contact with the cell surface. 

 Accordingly, the requiring and nonrequiring strains may differ by 

 secondary protein linkages that serve to maintain the fibers in a tightly 

 folded condition. 



E. Attachment to Noncellular Surfaces 



Phages attach, not only to specific host ceUs, but also to ion exchange 

 surfaces, such as glass and resins. In certain important respects attachment 

 to glass or a cation exchange resin closely resembles attachment to ceUs (Puck 

 et al., 1951; Puck and Sagik, 1953). For example, both reactions have similar 

 specific requirements for inorganic salts and, in the case of the tryptophan- 

 requiring strains of T4, for L-tryptophan. The tryptophan effect offers com- 

 pelling evidence that the same sites on the phage are involved in both re- 

 actions. Attachment of Tl to glass or resin is a reversible reaction, while 

 attachment of T2 to resin leads to the release of DNA from the protein 

 coat. 



There are differences as weU as similarities between the behavior of phage 

 toward ion exchangers and ceUs. For example, the salt requirement for attach- 

 ment of T2 to a cation exchange resin can be satisfied either by monovalent 

 or divalent cation salts, but only by the monovalent cation salts for attach- 

 ment to cells. The biological implications one associates with the attachment 

 of phage to ion exchange surfaces depend, of course, on whether greater 

 emphasis is placed on such points of difference between ion exchangers and 

 cells (Hershey, 1957a), or on the similarities (Puck et al., 1951; Puck and 

 Sagik, 1953; Puck, 1953; Tolmach, 1957). 



Phage also attach to an anion exchange resin. This reaction, in contrast 

 to the reaction with a cation exchanger or a cell, occurs as well in distilled 

 water as in a salt medium, showing that salts are required only for attach- 

 ment to a negatively charged surface. 



