240 BACTERIOPHAGES 



in mature, extracellular phage, and there is strong evidence 

 against the existence of the usual enzymes of intermediary 

 metabolism. This implies that the multiplying phage is de- 

 pendent on the host cell for energy yielding enzyme systems and 

 for the synthesis of the common organic compounds which are 

 constituents of mature phage. In addition the infected host cell 

 may synthesize certain new enzymes which are required for 

 phage replication and which are found in neither the uninfected 

 host cell nor in mature phage. Such hypothetical enzymes would 

 then be uniquely associated with the vegetative state of bacterio- 

 phage. 



b. Enzymic Activity Associated with Vegetative Phage 



The infected bacterium has a number of physiological proper- 

 ties not shared by the normal bacterium. It synthesizes several 

 antigenically specific proteins that are unique to the phage 

 particle, as well as a unique kind of nucleic acid. These proteins 

 and nucleic acid molecules are assembled into the peculiar 

 morphological structure characterizing the mature phage particle. 

 Nothing is yet known about the enzymes required for the syn- 

 thesis of proteins and nucleic acids, but if any of the specificity of 

 these substances is dependent on specific enzymes, then vegetative 

 phage must have associated with it some phage-specific en- 

 zymes. A search for such enzymes is beyond the reach of 

 present biochemical techniques. 



In the meantime, several pieces of evidence suggest the 

 possibility that there may be enzymes of intermediary metabo- 

 lism specifically associated with vegetative phage. One piece of 

 evidence stems from the discovery of the pyrimidine base 5- 

 (hydroxymethyl)cytosine (HMC) in phages T2, T4, and T6 by 

 Wyatt and Cohen. This base is not present in detectable 

 amounts in the uninfected host bacterium and has not yet been 

 found elsewhere in nature. Infection of the host cell with phage 

 T2 results in a prompt halt in the synthesis of cytosine and initia- 

 tion of the synthesis of hydroxymethylcytosine (Hershey, Dixon, 

 and Chase, 1953). It has been suggested by Cohen (1953b) that 



