THE BIOCHEMISTRY OF PLANT VIRUSES 111 



high-speed ceutrifugation. The material obtained had 1.44 % of phosphorus, 

 most of which was present as ribonucleic acid, and the sedimentation co- 

 efficient of the virus was usually small, being 74 S, the particles being 

 spherical (Lauffer and Eoss, 1940) or nearly so. The infectivity of the pre- 

 parations was very low, and it now seems likely that the preparations were 

 not mainly \arus. 



This virus has been investigated more recently by Bancroft and Kaesberg 

 (1958), who followed the general methods of Koss, and foimd that the 

 preparations contained tliree components with sedimentation coefficients of 

 73, 89, and 99 S, respectively. AU contained ribonucleic acid and aU were 

 serologically similar. Only the 99 S component is infectious however. The 

 other components look as if they might be fragments of the infectious com- 

 ponent, which is oval in form, having dimensions of 20 m/x, X 55 m/x. Only 

 14 % of the material is in the form of the 89-S substance while the other two 

 are in equal amounts. The most rational explanation of the results of Ross 

 and Lauffer is that their material was largely the 73->S component con- 

 taminated with a little 99-/S component. 



The alfalfa mosaic virus is unusual in that it is readily inactivated by 

 digestion with trypsin, as is potato virus X. 



XIII. Broad Bean Mottle Virus 



Bawden and associates (1951) have worked with a very unusual virus 

 wliich affects Vicia faba, the broad bean. Up to 2 gm. /liter of a nucleo- 

 protein can be obtained from infected plants, although the sap is not very 

 infectious itself. This is probably because the nucleoprotein itself has a very 

 low infectivity, about 10-*-10~^ gm./ml. being required for infecting plants. 

 This is nearly a milHon times as much as is required for viruses such as 

 tobacco mosaic, and the question arises whether the nucleoprotein is the 

 virus. Certainly the healthy beans do not contain material of the type and 

 the infective agent must have properties closely resembling those of the 

 nucleoprotein. The latter is remarkable, too, in that it needs 75 % saturation 

 with ammonium sulfate to cause the nucleoprotein to precipitate. This 

 simplifies the purification a great deal, because it is only necessary to take 

 the sap to half saturation with ammonium sulfate to clarify it, and then 

 add an equal volume of saturated ammonium sulfate solution to precipitate 

 the nucleoprotein. Purification is complicated by the large quantities of 

 brown pigment produced in the bean sap. 



A. Properties 



The nucleoprotein consists of spherical particles of 17 mju, diameter as 

 seen electron microscopically; they contain 1.6 % of P, at least 80 % of 

 which is present as ribonucleic acid. 



