RECENT STUDIES ON THE HOMOGENEITY OF TOBACCO 



MOSAIC VIRUS 



H. K. SCHACHMAN 



Virus Laboratory, University of California, Berkeley 

 Berkeley, California 



The homogeneity of tobacco mosaic virus particles has been the 

 subject of much study and controversy. Considerable attention has 

 been directed toward the establishment of the identity of infectivity 

 with the large rod-like particles isolated from virus diseased tobacco 

 plants. One point of view has held that the virus particles have differ- 

 ent lengths, varying from about 35 m[i to 1000 m|i, any one of which 

 or most of which possess infectivity. On the other hand, it has been 

 contended that there is a minimum length particle of about 300 m|i 

 which possesses this biological activity. These points of view have been 

 adequately reviewed in the past (Pirie, 1945; Lauffer, Price, and Petre, 

 1949); therefore, this discussion will concern itself with recent un- 

 published experiments designed to provide useful information relative 

 to this problem. 



Since it is now common practice to isolate tobacco mosaic virus and 

 other viruses by a series of cycles of alternate high- and low-speed 

 centrifugation, we might profitably consider the method briefly. It is 

 well known that the so-called differential centrifugation process is in- 

 efficient as a method of fractionating particles of differing sedimenta- 

 tion rates. It is to be expected, therefore, that the final solution of 

 "purified" virus obtained with the aid of conventional preparative 

 centrifuges would contain a distribution of particles which is related 

 to the distribution in the original juice expressed from the plants. 

 Differences would arise due to alterations of the particles caused by 

 aggregation and rupture, or denaturation and insolubilization of the 

 particles during the isolation procedure. Particles having sedimenta- 

 tion rates much larger and much smaller than those of the principal 

 components would be eliminated in the course of the cycles of alternate 

 high- and low-speed centrifugation. 



If, now, the final preparation contained particles of different sizes 

 it would be important to determine with a convection-free analytical 

 ultracentrifuge the sedimentation rate of the infectious principle in an 

 effort to correlate biological activity with a specific particle. Lauffer 

 (1943), using the separation cell in the ultracentrifuge, demonstrated 

 that the infectious agent had a sedimentation constant which was 



