14 



Degradation and structure of tobacco 

 mosaic virus 



H. FRAENKEL-CONRAT AND K. NARITA 



Virus Laboratory, University of California, 

 Berkeley, California, U.S.A. 



Thanks to electron microscopy the shape of the tobacco mosaic virus rod 

 is now so well known, as to require hardly any recapitulation. The 300 m/x 

 long particle consists to approximately 94% of protein and to 6% of nucleic 

 acid. The protein is generally regarded as forming the outer shell around the 

 nucleic acid cylinder, although recent X-ray data indicate that the nucleic 

 acid is deeply embedded in the protein, and may actually approach the 

 periphery where the surface is deeply grooved. Within the past two years 

 it has clearly been demonstrated that the nucleic acid alone can initiate 

 infection and that it carries all the necessary 'information' for the produc- 

 tion of complete progeny virus. ^'^ The efforts of virologists are, therefore, 

 now centering on this crucial minor constituent of the virus. However, the 

 role played by the protein moiety in various important regards must not be 

 overlooked. In quantitative respects, the protein contributes to viral infec- 

 tivity, since it potentiates the action of the nucleic acid 20-50 fold, possibly 

 by giving it protection and rigidity.*-^ The protein also accounts for the 

 serological activity of viruses and thus gives us a tool with which to combat 

 them. And finally, the protein probably influences host range specificities and 

 virulence. It is of particular interest that the protein components have been 

 shown to differ in specific and characteristic manner from one virus strain 

 to another.^'' The protein may, therefore, be regarded as a phenotypic 

 expression of the chemically still obscure genotypic characteristics of viruses. 

 This symposium has clearly documented our advances towards the goal of 

 complete structural analysis of proteins. The possibility exists, therefore, 

 that the structure of the virus protein may be elucidated within the next 

 few years. The virus may then become the Rosetta stone of biochemical 

 genetics, in supplying the bilingual record needed to decipher nucleic acid 

 in terms of protein structure. 



While various estimates of the virus particle weight range from 40 to 

 50 million, it has been recognized for several years that each rod represents 



