LIFE S FRINGES 



Stanley that similarly favour this view. In the first place it has been 

 unambiguously shown that numerous treatments, in part very mild 

 ones, that deprive a solution of the crystals of its activity as a virus, 

 also cause more or less radical changes in the protein. Separation of 

 protein and virus by ultrafiltration appeared impossible ; if the pore 

 size of the filter was small enough to retain the protein, the filtrate 

 was invariably inactive. 



Particularly convincing are the results published in a recent com- 

 munication showing that during ultracentrifugation of solutions of the 

 crystalline protein the infectivity disappears from the upper layers at 

 the same rate as that at which the protein is removed as a consequence 

 of the sedimentation. 



The study of the 'aucuba' variety of the mosaic virus has also 

 yielded a crystalline protein that differs merely in some minor details 

 from the typical mosaic virus. Inoculation of tomato plants with the 

 tobacco mosaic virus also produced a crystalline protein that appeared 

 to be identical in every respect with the protein isolated from diseased 

 tobacco plants. An important additional fact is, furthermore, the dem- 

 onstration by Bawden and co-workers that the juice from healthy to- 

 bacco plants does not contain this protein. Moreover, Stanley and 

 Wyckoff have recently reported that by means of alternating centrifu- 

 gation and ultracentrifugation they have succeeded in isolating, again 

 in crystalline form, the very unstable causal agent of a third virus dis- 

 ease of tobacco, the so-called ringspot disease. This, too, appeared to 

 be a protein; it differed markedly from the mosaic virus in its chemical, 

 physical, and serological properties, as could have been expected. 



In view of this assembly of facts the identity of viruses and the iso- 

 lated proteins appears irrefutable. It is therefore important to find out 

 what has become known concerning the chemical nature of these pro- 

 teins. The X-ray diagrams of the mosaic and aucuba virus, determined 

 by. Wyckoff and Corey, agree in essence completely with those of ear- 

 lier investigated proteins. Sedimentation analysis with the aid of the 

 ultracentrifuge showed convincingly that the molecular weight of the 

 virus protein is very large, and exceeds 10,000,000. Svedberg, in a 

 later investigation, obtained a value of 17,000,000. The significance 

 of this fact becomes clear if viewed in conjunction with the fact that 

 the juice of healthy tobacco plants does not contain any proteins with 

 a molecular weight over 30,000 in appreciable amount. In dilute 



34i 



