( J6 Essays in Biochemistry 



analysis of the protein component of mutant strains of tobacco mosaic 

 virus, one might expect that these will differ qualitatively and quanti- 

 tatively from phage type to phage type. The amino acid content of 

 T 3 prepared by a variety of procedures has been shown to remain 

 the same and is apparently a specific and unchanging property of the 

 phage particle. There is nothing in the amino acid distribution to 

 suggest any specific characteristics for the coliphage proteins, although 

 the dicarboxylic acids are present in unusually large amounts. 



So far as nucleic acid content is concerned, the coliphages contain 

 only DNA. The most recent values for the purine and pyrimidine 

 base content are listed in Table 1. The DNA of T 2 , T 4 , and T (; is 



Table 1. Composition of Coliphage DNA 



5-Hydroxy- 

 Virus Adenine Thymine Guanine Cytosine methylcytosine 



moles/100 moles estimated bases 



19.5 



16.7 

 17.0 

 16.7 



27.1 



* G. R. Wyatt, Cold Spring Harbor Symposia Quant. Biol., 18, 133 (1953). 

 f C. A. Knight and D. Fraser, in C. A. Knight, Advances in Virus Research, 

 II, 168 (1954). 



| J. D. Smith and L. Siminoviteh, in A. Lwoff, Baderiol. Revs., 17, 320 (1953). 



entirely unlike the nucleic acid from any other source. In these phages, 

 cytosine is absent and one finds a new pyrimidine base, 5-hydroxy- 

 methylcytosine (5-HMC). In addition, glucose is present, one mole- 

 cule of the hexose being attached probably to the hydroxyl group of 

 each 5-HMC molecule. However, 5-HMC is not present in the odd- 

 numbered phages (Table 1 ), and the question of the possible presence 

 of glucose has not been examined. 



The manner in which the nucleic acid and protein of the coliphage 

 particle are bound together is not known. It is generally believed, 

 however, that the nucleic acid is completely sheathed by a protective 

 protein layer. If one rapidly dilutes saline suspensions of some of 

 the coliphages, the nucleic acid is liberated into the medium and the 

 remaining protein "ghosts" are seen, under the electron microscope, to 

 retain the spermlike shape of the intact particle although the hexagonal 

 head is empty. 



