STRUCTURAL AND CHEMICAL ARCHITECTURE OF HOST CELLS 25 



C. Possible Contributions of the Host Cell to the Synthesis of Viral 



Polymers 



Despite the existence of such virus enzymes, the host is still called upon 

 to provide the metabolic equipment essential to the synthesis of the amino 

 acids and nucleotides, which are then incorporated into the specific viral 

 polymers. How are specific viral polymers synthesized? To what extent do 

 the viruses organize their own duphcation? It can be assumed that the models 

 provided by the infecting virus particle in some mamier determine the 

 specificity of the viral polymers finally produced. However, it was noted only 

 some five years ago (Cohen, 1952) that biochemists were not then in a position 

 to state that viruses do not contain enzymes for the specific organization of 

 nucleic acid from nucleotides or of proteins from amino acids. At that time 

 we did not know how to test for such enzymes, nor did we know if enzymatic 

 activities were required for the specific organization of these particular 

 polymers. We do not yet have answers to the first of these questions. Never- 

 theless, it is now known that the biosynthesis of nucleic acids and protein do 

 require specific proteins and it may be anticipated that tests will be made in 

 the near future to determine whether the viruses contain the enzymes for 

 the synthesis of their own polymers, i.e., to see if they are truly "self- 

 duplicating." It wUl be a major function of this article to detail the rapid 

 growth of biochemical knowledge in the areas relevant to the problem of the 

 synthesis of viral polymers. 



Two lines of evidence suggest that the enzymes of the host cell are also 

 used for polymer synthesis: 



1. The first relates to the apparently unique case of the production of 

 5-hydroxymethyl cytosine. As of this writing, the enzyme for the formation 

 of the deoxyribotide of HMC by the hydroxymethylation of deoxycytidyhc 

 acid in the presence of 5, 6, 7, 8-tetrahydrofohc acid (see Fig. 1) has been 

 found only in extracts of bacteria infected by the T-even phages (Flaks and 

 Cohen, 1957). Since the enzyme does not appear to be present within virus 

 and hence is probably not introduced with virus DNA, the following possi- 

 bihties concerning its origin are now beginning to be explored: 



a. The enzyme is inhibited in uninfected cells and this inhibition is 

 released upon infection with virus, or 



b. The enzyme is synthesized after viral infection: 



i. The synthesis of the enzyme is induced by low molecular weight frag- 

 ments of injected virus DNA containing hydroxymethyl cytosine. 

 ii. The synthesis of the enzyme is linked to the presence and perhaps the 

 metabolism of a specific polynucleotide sequence derived from the injected 

 viral DNA. 



Of the formal possibilities which are indicated above, it can be seen that 

 "bi" and "bii" call for the participation of the host ceU m polymer synthesis 



