12 F. M. BURNET 



TABLE I 

 Nucleic Acid Content of Six Viral Groups 



DNA and RNA L-NA RNA 



Psittacosis group Poxvirus (?) group Myxovirus group 

 Adenoviruses Arborviruses 



Enteroviruses 



been concerned with. The possibility that poxvirus should be in the first 

 (DNA and RNA) group is not wholly excluded. 



The most significant chemical finding relevant to the point at which inter- 

 ference by a virus can occur is derived from the study of the action of the 

 ribosyl-benzimidazole derivative, DRB. This inhibits the early stage of 

 influenza virus production and shows an inhibitory influence on the uptake 

 of labeled amino acids by thymus nuclei with almost the same time sequence. 

 In the latter case, Mirsky and Allfrey (1957) consider that the effect is to 

 block the synthesis of RNA, which is needed before protein synthesis is 

 possible. Once RNA of appropriate specific pattern has been synthesized, it 

 appears that protein synthesis can go on without the necessity for concomit- 

 ant further synthesis of RNA. 



One way of looking at the RNA-containing viruses could well be to concen- 

 trate on the ribonucleoprotein common to them all. There is very much to 

 suggest that, as Poison and others have claimed, soluble complement-fixing 

 antigens from viruses tend to be about the same size, 10-15 m/x, as is postu- 

 lated for the primary protein synthetic mechanism by electron microscopists. 

 Pallade (1957) for instance, considers that spherical units of RNA and protein 

 about 150 A in diameter, represent a standard form of protein synthetic 

 mechanism in animal, plant, and bacterial cells. A poliovirus particle could 

 be pictured, according to Crick and Watson (1956), as a firm, symmetrical 

 structure of 12 such units (or 60 smaller ones), containing only ribonucleic 

 acid and protein. Western equine encephalitis virus may represent a similar 

 complex which, in the process of formation, takes up a large proportion of 

 lipid, and influenza virus as a loose accumulation of about the same number 

 of primary units contained in an enclosing membrane of complex character 

 which incorporates both viral protein and host cell constituents. 



On this view, the essential interference by the virus might well be the 

 intrusion of virus RNA into the nucleus where virus RNA-protein complexes, 

 which in the appropriate metabolic environments can produce either RNA 

 or protein, are laid down. The possibility that the new synthetic units are in 

 some fashion contrived from cell units already with a defined function might 

 allow for the synthesis of viral complexes or components with the same sort 



