20 VIRUSES 



of water. Taking this into account, the wet molecular weight of 22 million 

 would correspond to a dry molecular weight of about 12 million. This ralue is 

 in reasonably good agreement with that obtained from diffusion and sedimenta- 

 tion, 10.6 + 1 million. Also, the amount of water computed from X-rsiy data 

 agrees with that obtained from sedimentation and diffusion data. Very recently, 

 Oeter used the method of light scattering to determine the molecular weight of 

 bushy stunt virus. The principle of this method is that the amount of light 

 scattered by a colloidal solution of spherical particles is proportional to the 

 size of the particle. Oster obtained a yalue of 9 million for the anhydrous 

 molecular weight of bushy stunt virus. Thus there are three estimates of the 

 molecular weight of bushy stunt virus, 9 million from light scattering, 10.6 

 + 1 million from sedimentation and diffusion, and 12 million from X-ray studies. 

 The average would be 10.5 million. iTurther, the electron microscope shows 

 that the particles are spheres. X-ray data indicate that the spheres in the 

 crystals are 6?% hydrated and sedimentation and diffusion data indicate that 

 they are 77?^ hydrated. JJ'rom the average molecular weight, 10.5 million, and 

 the dry density, one can calculate that the diameter of the dry virus particle 

 should be 29 millimicrons. Price, Williams and Wyckoff made niunerous measure- 

 ments of the diameter of the bushy stunt virus particles on electron micro- 

 graphs. They obtained values ranging from 25 to 27 millimicrons. Thus we can 

 see how the physical picture of the bushy stunt virus was built up. We know 

 that it is a sperical particle with a dry molecular weight between 9 ^nd 12 

 million and a dry diameter between 25 and 29 millimicrons, but that in solu- 

 tion, it contains between 67 and 77^ water of hydration. 



Pictures of the crystals of southern bean mosaic virus, recently crystal- 

 lized by Price, and a gold shadow micrograph are shown in Figures 10 and 11. 

 It is evident that the southern bean mosaic virus is also essentially spherical. 

 Miller and Price have measured several of its physical constants. The sedimen- 

 tation constant is t.0"13 cm. per sec. per unit field, the diffusion constant 

 is 1.34 X 10"7 cm.^/sec. and the partial specific volume is O.70. Jf'rom these 

 data, again as in the manner employed in the case of bushy stunt virus, one can 

 calculate that the southern bean mosaic virus is a hydrated sphere with dO% 

 water of hydration, with a dry molecular weight of 6,630,000, and a dry radius 

 of 26 mu. 



Tobacco Mosaic Virus 



We come now to a consideration of tobacco mosaic virus. A study with 

 this virus was carried out at the Rockefeller Institute Laboratory with a sin- 

 gle preparation. It was photographed with the electron microscope and shown 

 to consist of rod-like particles with an average length of 270 millimicrons, 

 as shown in i^igure 5« '^'he thickness of the virus particles as obtained from 

 the electron micrograph is about 15 millimicrons. However, a more precise 

 figure was obtained from X-ray diffraction studies carried out by aernal and 

 tfankuohen. These investigators found that the virus rods are lined up paral- 

 lel to each other with hexagonal symmetry with respect to cross section, as 

 illustrated in Figure I6. 



