208 BACTERIOPHAGES 



1. Morphological, Functional, and Chemical Differentiation of the 

 Phage Particle 



A most important discovery relative to the fate of the infecting 

 phage particle was reported by Hershey and Chase (1952) and 

 further elaborated by Hershey (1955). These experiments have 

 been described in detail in Chapter XI. The essential features 

 together with other relevant details will be briefly summarized 

 here. Most of the information has been obtained with the T2 

 serological group and with T5, and it may be unwise to ex- 

 trapolate at present to other kinds of phage. 



a. Chemical Composition 



Phage T2 consists almost exclusively of protein and deoxy- 

 pentose nucleic acid (DNA). Few attempts have been made as 

 yet to fractionate the proteins of phage but serological evidence 

 suggests that at least two different proteins are present, one in the 

 phage head and a second in the phage tail. The protein con- 

 tains a considerable amount of methionine and so can be con- 

 veniently labeled with S^^. The DNA may be labeled with P^- 

 and both protein and DNA can be labeled with C and N isotopes. 



h. Physical Fractionation — Osmotic Shock 



Phage T2 consists of a protein membrane and a DNA core. 

 By means of osmotic shock followed by high speed centrifugation 

 it is possible to separate the sedimentable membranes from the 

 DNA in the supernatant fluid. Not more than 5 per cent of the 

 total phage protein remains with the DNA fraction. Even 

 this protein has the same amino acid composition as the sedi- 

 ment. About 3 per cent of the total sulfur-containing protein 

 of the phage may enter the host cell with the phage DNA. The 

 function of this protein is not known. This does not leave much 

 room for a hypothetical, sulfur-free, basic protein which might 

 be in intimate association with the liberated DNA. If such a 

 protein exists it must amount to less than 5 per cent by weight of 

 the phage DNA (Hershey, 1953c). Therefore phage T2 must 



