318 H. K. SCHACHMAN AND R. C. WILLIAMS 



The data can be reconciled with a sphere containing large amounts of water 

 within it or an anhydrous, elongated particle but the latter is precluded as a 

 result of the results of X-ray diffraction, low-angle X-ray scattering, and 

 electron microscopy. Also the difference in volume between air-dried and 

 frozen-dried particles is evidence for internal hydration. Finally the mole- 

 cular weight, partial specific volume, and diameter of the particles in solution 

 cannot be reconciled without invoking a spongelilce model which contains 

 water in its interstices. The exact amount of water is still uncertain. How- 

 ever, reliable estimates can be expected as the data from individual lines of 

 evidence become more accurate. 



As yet there is only scant physical chemical information regarding the 

 internal structure of the virus particles. X-ray diffraction studies show, of 

 course, a high degree of order within the macromolecules. Kecently the 

 diffraction patterns have been interpreted in terms of subunits of protein 

 surrounding a core of RNA (Crick and Watson, 1956; Caspar, 1956a). Caspar 

 suggested that the number of subunits is a multiple of 60. Experimental 

 evidence pertinent to this question is provided by experiments of Hersh 

 and Schachman (1958) who showed that the virus can be degraded into much 

 smaller units by the action of sodium dodecyl sulfate at low temperature 

 and mildly alkaline pH. Since this treatment is not likely to cause the 

 rupture of covalent bonds, the isolated fragments can be considered as real 

 structural subunits. Separation of the protein from the RNA followed by 

 examination of the protein by the Archibald method gave 6 X 10* for its 

 molecular weight. This value is compatible with the chemical evidence based 

 on end-group analysis (Niu et al., 1958) and amino-acid composition 

 (deFremery and Knight, 1955). No reports have appeared, as yet, regarding 

 the size of the RNA. 



In conclusion, bushy stunt virus appears to be a very highly organized 

 particle composed of a central core of RNA surrounded by protein molecules 

 which are held together by specific, secondary forces. The packing of these 

 molecules provides for interstitial spaces, creating a rigid spongelike struc- 

 ture within which water is immobilized during the movement of the particles 

 in solution. Apparently the spongelike structure possesses considerable 

 rigidity, for removal of the water by freeze-drying does not cause the 

 collapse of the structure. 



3. Turnip Yellow Mosaic Virus 



Although research with turnip yellow mosaic virus (TYMV) has not been 

 as extensive as that with BSV, the limited investigations provide a fascinat- 

 ing tale. These studies, conducted principally by Markham and K. M. 

 Smith (1946, 1949) who first isolated this virus, have been extremely reward- 

 ing. As a result TYMV serves as the principal model on which are based 



