THE INITIATION OF BACTERIOPHAGE INFECTION 211 



in a suitable medium, lead to the formation of a dense spherical body, called 

 a protoplast, which retains most of the synthetic activities of the intact cell. 

 A protoplast lyses when suspended in water, leaving an empty spherical 

 casing that appears to represent its limiting membrane. The protoplast 

 membrane, imlike the cell wall, does not bind phage (Weibull, 1953a; 

 Brenner and Stent, 1955; Zinder and Arndt, 1956); the fact that some proto- 

 plast preparations do bind phage probably can be accounted for by residual 

 wall material. Analysis of the membrane from Bacillus megatherium shows 

 that it is composed largely of hpid, protein, and pigments, and contains 

 several enzymes, including the entire cellular cytochrome system (WeibuU, 

 1953b; Storck and Wachsman, 1957). Thus the cell membrane differs from 

 ceU wall in morphology, chemical composition, sensitivity to rupture under 

 conditions of high osmotic pressure, and ability to bind phage. 



There has been detected in cell walls a subunit composed of a pentapeptide 

 containing alanine, glutamic acid, and lysine (or diaminopimehc acid) linked 

 to the amino sugar, N-acetylmuramic acid. Cells of Staph, aureus treated 

 with penicillin excrete this compound in the form of a uridine diphosphate 

 complex, which suggests that protoplast formation with penicillin results 

 from the inability to incorporate the subunit into new ceU wall material 

 (Park and Strominger, 1957). The subunit can be removed from the cell wall 

 of E. coli by an enzyme present in the tail of T2 (Weidel and Pringosigh, 

 1957). This reaction may provide the means for penetrating the cell wall 

 barrier in the course of infection (see Section VII). The membrane also 

 presents a barrier to the infecting phage, but there is no information about 

 the reactions employed ia penetrating this layer. 



IV. Injection 



The first clear indication that different functions were performed by 

 different parts of a phage particle came from an experiment by Novick and 

 Szilard (1951). In examining the phage issuing from cells infected with both 

 T2 and T4, it was found that some of the particles had imusual properties. 

 The host range of these particles (see Section VI) corresponded to that of 

 T4, but all of the progeny produced by these particles exhibited the host 

 range characteristics of T2. It could be concluded that the phage component 

 responsible for host range specificity did not also serve as its own genetic 

 determinant. 



The chemical identity of the genetic component was revealed by the 

 blendor experiment of Hershey and Chase (1952). It had already been shown 

 for T2 that its DNA was contained in a protein coat, and that attachment 

 occurred at the tip of a long tail on the coat. Hershey and Chase had reason 

 to believe that, during infection, only the DNA, and not the protein coat, 

 entered the cell. In order to test this possibiHty, they infected cells with T2 



