206 A. GAEEN AND L. M. KOZLOFF 



2. Chemical Composition 



The most extensive chemical analyses on a phage have been carried out 

 with T2. The total weight per infective particle is 5 X 10~^^ grams, consisting 

 mostly of protein and deoxyribonucleic acid (DNA) in about equal amounts. 

 There is no ( < 0.25 %) ribonucleic acid and no ( < 0.5 %) lipid (Cohen, 1947; 

 Herriott and Barlow, 1952; Volkin and Astrachan, 1956). The presence of 

 DNA is common to all phages studied (coliphages Tl, T2, T3, T4, T5, T6, T7, 

 lambda, and Salmonella phage P22), generally in amounts proportional to 

 the relative volumes of the particles (Stent, 1958). (There exist differences in 

 the composition of theDNA of different phages, which are discussed elsewhere 

 in this volume.) Probably the absence of ribonucleic acid also is a common 

 .characteristic of phages; T7, like T2, contains no RNA (Lunan and Sinsheimer 

 1956), and it is not unlikely that the small quantities of RNA reported in 

 preparations of other phages (and in other preparations of T2 and T7) were 

 residual bacterial materials (Putnam, 1953). About 95 % of the phosphorus 

 of T2 (Herriott and Barlow, 1952), but only 71 % of the phosphorus of T7 

 (Lunan and Sinsheimer, 1956), is located in DNA. The nature of the non- 

 DNA phosphorus fraction is not known. 



A few per cent of the material of T2 is soluble in cold trichloroacetic acid 

 (TCA). Three components have been identified in this acid-soluble fraction: 

 a peptide containing only aspartic acid, glutamic acid, and lysine (in contrast 

 to the protein of intact phage, which contains at least 16 amino acids), and 

 two free amino acid-like substances (ninhydrin-positive and probably basic) 

 that are chromatographicaUy different from aU amino acids tested (Hershey, 

 1955, 1957b). ^ These components represent about 1 % of the phage carbon. 



3. Organization of Protein and DNA 



An important clue to the structure of phages was provided by the discovery 

 that the T-even phages lost infectivity when rapidly diluted from concen- 

 trated salt solution into water (Anderson, 1949; Anderson et al., 1952), and 

 that, concomitantly, DNA dissociated from the protein (Herriott, 1951). 

 This effect is called osmotic shock, since gradual dilution is ineffective. Electron 

 micrographs of shocked preparations contain particles that retain the over- 

 all shape of infective particles but in which the head appears to be collapsed 

 and empty, hence the name ghosts (Fig. 2). There also are networks of DNA 

 fibers lying outside of the ghosts (Williams, 1953). In contrast to the DNA 

 in infective particles, the DNA of shocked preparations is sensitive to de- 

 gradation by DNAase. Ghosts purified by centrifugation after DNAase treat- 

 ment contain most of the phage protein and are devoid of DNA (Hershey, 



^ These two substances have recently been identified as spermidine and putrescine 

 (Ames et al., 1958). 



