THE PROBLEMS OF VIROLOGY 7 



a convenient way of generalizing the situation until increasing knowledge 

 allows a more subtle appreciation. In the case of tobacco mosaic virus, the 

 specific viral protein seems to have the quality of building up a helical 

 structure to enclose ribonucleic acid threads of almost any type, even the 

 semisynthetic polynucleotides produced by Ochoa's methods (Hart and 

 Smith, 1956). However, such products are far less stable than the structures 

 formed with homologous protein and nucleic acid. Although it seems unlikely, 

 it may be that the only functionally important character of TMV protein is 

 its geometrical form and what other attributes are necessary to allow orderly 

 aggregation into the characteristic hollow helices. If this is so, less may be 

 obtained from strictly chemical studies of amino acid content and sequence 

 than from physical and topological analysis of the secondary coiling and 

 organization of the polypeptide chain. It is stiU open, however, that the 

 protein surface of a plant virus has a specific function in facilitating entry of 

 virus into cells not previously infected. In the case of the bacterial viruses, 

 the organization of the protein coat seems to be much more elaborate than 

 in a typical plant virus and a number of functionally distinct regions are 

 recognized. It is now generally accepted that the tip of the tail is a highly 

 specialized structure the terminal fibers of which serve as a specific anchoring 

 mechanism while another portion is exposed or activated so as to act enzymat- 

 ically on components of the bacterial wall. There are huits that the smaller 

 animal viruses may have components, presumably protein, with specific 

 adsorptive qualities for the cells they infect. The poKo-hke virus GD Vll ag- 

 glutinates human red cells. TheUer's virus is inhibited by a polysaccharide 

 from mouse intestine. These may be accidental findings of no biological sig- 

 nificance, but they point in a direction where further study may be fruitful. 



The larger viruses are clearly beyond the macromolecular level and for the 

 present at least must be regarded as primitive organisms. Chemical study will 

 therefore be limited to the analysis of proximate constituents without any 

 serious hope of providing a meaningful picture of the virus particle at the 

 purely chemical level. Basically the same methodology must be applied as to 

 the morphological and chemical study of any small microorganisms. In the 

 larger virus types there will automatically be greater opportmiities for rela- 

 tively mimodified host materials to be present in the particles. It is probably 

 too naive to use terms like "pure virus" or "contamination with host material" 

 at this level. If one view of the structure of influenza virus is correct, essen- 

 tially unmodified host components form a very significant part of the virus 

 surface. In each case the nature of components which are not identifiable as 

 of specifically viral quality will have to be judged on what biological and 

 other evidence is available. 



The question of enzymes in relation to viral particles has been widely dis- 

 cussed. Only in the influenza and related viruses of the myxovirus group has 



