THE INITIATION OF BACTERIOPHAGE INFECTION 207 



1955). These properties indicate that the heads of intact particles are semi- 

 permeable protein coats that encase the DNA and protect it from enzymatic 

 degradation. The release of DNA by osmotic shock appears to occmr through 

 a rupture in the head protein (Fig. 2). 



The materials released from the coat of shocked T2 include, in addition to 

 DNA, a protein amounting to about 2 % of the total phage carbon (with 

 about the same amino acid composition as the protein of the intact particle 

 but perhaps richer in lysine), and the three TCA-soluble components de- 

 scribed in the preceding section (Hershey, 1955, 1957b). 



The viabihty of phages outside the T-even group is not affected by osmotic 

 shock. This insensitivity to osmotic shock probably is not due to the absence 

 of a protein coat, since DNA-free ghosts of T5 can be formed by heating in a 

 medium deficient in divalent cations (Lark and Adams, 1953), and ghosts of 

 Tl, T5, PI, lambda, and a staphylococcal phage have been seen in electron 

 micrographs (Anderson, 1953; Hotchin, 1954; Kellenberger and Kellenberger, 

 1957). The coats of insensitive phages may be either impermeable or freely 

 permeable to salts, or perhaps capable of withstanding high osmotic 

 pressures. 



4. Structural Units 



Much of our present knowledge about the structure of phage coats has come 

 from high resolution electron micrographs of T2 particles disrupted by freez- 

 ing and thawing (Wilhams and Fraser, 1956) or by treatment with a solution 

 of HgOa-ethanol (Kellenberger and Arber, 1955). The following components 

 have been detected. (Fig. 3): intact heads (tailless but retaining DNA), ghost 

 heads (devoid of DNA), intact tails, shortened tails with a protruding core 

 100 A thick, and tail fibers 60 A thick which either are free or attached to 

 the tip of a tail or a core. Treatment with DNAase does not alter these 

 forms. 



Some of these components are also visible in electron micrographs of crude 

 lysates of T2-infected cells (Fig. 4). It is possible to identify intact phage, 

 heads containing DNA, ghost heads, and in addition, a component called a 

 rod, whose length corresponds to that of the tail (1000 A) and thickness to 

 that of the core found protruding from the tail of partially disrupted particles 

 (Fig. 3). The rods frequently have fibers attached. It is likely that the rods 

 represent cores that have been released from (or not yet inserted into) tails. 

 Similar rods were first identified in preparations of a staphylococcal phage 

 (Hotchin, 1954). 



A current conception of the organization of components in a T2 particle 

 is diagrammed in Fig. 5. It is already apparent that there is considerable 

 anatomical sophistication to this virus. 



