THE BIOCHEMISTEY OF PLANT VIRUSES 95 



uncertainty because of the glycine arising from the breakdown of the purines 

 of the ribonucleic acid. 



Caspar (1956a) has pointed out that the tryptophan, cysteine, methionine, 

 and histidine are in the proportion of 1:2:2:3 residues in a molecular 

 weight of 25,000 to 28,000, which he proposes as the size of the protein 

 subunit of the virus, 



C. Physical Properties 



The virus dissolves in aqueous solvents to give opalescent solutions which 

 are isotropic. The isoelectric point of the virus is at pH 4.1 (McFarlane and 

 Kekwick, 1938), and the virus is soluble over a wide range of pH's. The 

 ultraviolet absorption shows a peak at 262 m/x, and the extinction of a 

 O.l-mg./ml. solution is 0.52 for a 1-cm. layer (Bawden and Pirie, 1938b). 



The molecular weight from light scattering is 9,000,000 (Oster, 1946); 

 this is in good agreement with results obtained by other methods, which 

 are summarized by Markham (1953b), and include a direct particle count 

 (Williams and Backus, 1949). The sedimentation coefficient is 131 S (Lauffer 

 and Stanley, 1940), the earher value of 146 S given by McFarlane and 

 Kekwick (1938) having been in error owing to a defect in the hydrogen- 

 cooling supply in the Svedberg equilibrium centrifuge used for the measure- 

 ment (E.. A. Kekwick, personal cormnunication). Much of the early work on 

 the size and molecular weight of this virus is discussed at length by Markham 

 et al. (1942). 



Bushy stunt virus was one of the earliest viruses to be investigated by 

 X-ray crystallographic teclmiques. The original experiments were made by 

 Bernal et al. (1938); (Bernal and Fankuchen, 1941b) who used powders of 

 small crystals, and examined them both wet and dry. The wet crystals 

 gave reflections which were consistent with a body-centered cubic lattice 

 of side 394 A. On drying the crystals shrink, and the crystal spacings also 

 shrmk; in the dry crystal the ordy spacing obtained indicated that the cell 

 size had decreased to a 318 A cube. This interesting work indicates that the 

 virus is itself hydrated and shrinks on drying, because the average diameter 

 of the particles deduced from the diffasion coefficient of Neurath and Cooper 

 (1940), 1.15 X 10~' cm.2/sec., is 37 m/x. A body-centered cube having rigid 

 spherical particles of this size in point contact would have an edge of 42.7 m/x, 

 so that the particles must be in contact over a flattened area and have an 

 effective diameter 92.5 % of that of the equivalent spheres. (This is what 

 one would expect if the particles were polyhedral. The smallest diameter of 

 an icosahedron is about 81 % that of a sphere of equal volume.) The anhy- 

 drous particles having a molecular weight of about 10 million, if they were 

 the same shape as the hydrated particles, would give a cube of side 310 A. 

 The agreement between these figures is a strong indication that the particles 



