96 R. MARKHAM 



swell reversibly on wetting. The disorder, evidenced by the loss of reflexions 

 on drying the crystals, is paralleled to some extent by a loss of infectivity 

 of the virus by incautious drying. Infectivity may be retained on drying by 

 very slow drying over solutions of relatively high vapor pressure, such as 

 saturated zinc sulfate in water. 



Stationary single crystal X-ray diffraction measurements were made by 

 Carlisle and Dornberger (1948), who obtained similar but slightly smaller 

 cell sizes, the wet cell being 368 A and the dry one 314 A. It is, of course, 

 possible that the wet cell size will vary with the suspending fluid, the powder 

 photographs having been taken with the crystals fully immersed in the 

 mother liquor, whereas single crystals are usually drained. 



The X-ray structure has been reexamined more recently by Caspar (1956a), 

 using single crystal precession photographs, and he deduced that the virus 

 particles themselves possessed cubic symmetry, and were built up from 

 60 equivalent subunits, or a multiple of 60 subunits, as has been suggested 

 by Crick and Watson (1956) from theoretical considerations. As mentioned 

 earlier, this idea is compatible with the analytical data which have been 

 published. Caspar thinks that the number of protein subunits involved is 

 probably 360. 



Bushy stunt virus crystallizes in other forms in the presence of substances 

 such as heparin (Cohen, 1942), but these crystals have not been examined 

 in any detail so far. Superficially, they look as if they do not belong to the 

 cubic system. 



VIII. The Tobacco Ringspot Virus 



This virus, which is a common disease agent of tobacco and a number 

 of other plants, receives its name from its characteristic of causing the 

 formation of concentric rings on many of its hosts (Fig. 10). This tendency 

 to form rings is one of the characteristics of plant viruses, but the mechanism 

 involved is uncertain. 



In 1939 Stanley reported the isolation of tobacco ringspot virus from 

 Turkish tobacco plants (Stanley, 1939). This virus is present in such plants 

 in small quantity only, and the isolation was performed by high-speed 

 centrifugation because much of the activity of the virus is lost when ammon- 

 ium sulfate is used for the precipitation. The material which was obtained 

 was fairly unstable at room temperature, but was stable at 4°C. It contained 

 a very large amount of nucleic acid, estimated from the phosphorus content 

 as 40 %, a substantial proportion of which was isolated as (somewhat 

 degraded) ribonucleic acid by the method of Johnson and Harkins (1929). 

 The virus had a sedimentation coefiicient of 115 S, and appeared to be 

 essentially spherical from the viscosity and from the lack of anisotropy 



