VIRUSES AS MOLECULES 



13 



p 6.2 63 6.4 6.5 6 6 



DISTANCE FROM AXIS OF ROTATION IN CENTIMETERS 



FIGURE 9 - BOUNDARY SPKEADIITG DT SEDIUENTATIOIT EXPERIMENT OF 

 BUSHY STUNT VIRUS. OPEN CIRCLES ARE EXPERIMENTAL POINTS OBTAINED 

 BY LAim SCALE METHOD. SMOOTH CURVES ARE THEORETICAL BOUNDARIES 

 CALCULATED FROM KtTOWN DIFFUSION CONSTAl-TT. (M-A.Lauf f er, J. Biol. 

 Chem. 149, 99 (1942) ). 



The curves seen there are simply a convenient way of describing quantitatively 

 the position and the sharpness of a boundary in the ultracentrifuge after vari- 

 ous periods of centrlfugation. They tell us that after 8? minutes of centri- 

 fugation the virus - buffer boundary had moved to the first position, and after 

 220 minutes it had moved to the last position. The sharpness of these peaks 

 tell us how sharp the boundary is at every stage. At first, it is fairly sharp, 

 then it becomes more diffuse. The points represent the actual sharpness of the 

 boundary in the centrlfugation experiment. The curves represent the theoretical 

 sharpness that one should get when the boundary spreading is due 100^4 to normal 

 diffusion and not all to Inhomogeneity . One can see perfectly well that the 

 agreement between theoretical and actual boundary sharpness is very good indeed. 

 This means, then, that the bushy stunt virus particles are absolutely homo- 

 geneous with respect to size and shape, or very nearly so. 



Through the study of tobacco mosaic virus, it was possible to demonstrate 

 that viruses are protein particles and that these particles are homogeneous 

 chemically and identical in one dimension. However, it was not possible to ob- 

 tain preparations in which the rod-like particles were all of exactly the same 

 length. Hence, we failed on the very last step in our attempt to prove that 

 tobacco mosaic virus particles are protein molecules. When bushy stunt virus 

 protein was considered, it was possible to show with the ultracentrifuge that 

 virus particles can all be of identical size and shape, thereby hurdling the 

 last serious barrier to believing that viruses are indeed protein molecules . 



Nevertheless, whether or not any viruses actually are molecules is a 

 question which may never be settled. It was sufficient for the cause of science 

 that the hypothesis that the viruses are molecules is not untenable, for this 

 idea has been of the most tremendous signif icauice . It has provided the inspir- 

 ation for an entirely new approach to virus research, yor forty years after 

 the discovery of viruses, very little real progress was made in the elucidation 

 of their nature. Almost all of the knowledge concerning the nature of viruses 

 that we have today has been developed during the past ten years, since the con- 

 cept of the molecular nature of viruses was given impetus by the crystallization 

 of tobacco mosaic virus. Numerous viruses have since been obtained in crystal- 

 Ine form. These Include many strains of tobacco mosaic virus, tomato bushy 

 stunt virus, tobacco necrosis virus, southern bean mosaic virus, and turnip yel- 

 low mosaic virus. A photomicrograph of crystals of the bean mosaic virus and 

 an electron micrograph of the particles of this virus are shown in Figures 10 

 and 11. 





