THE BIOCHEMISTRY OF PLANT VIRUSES 101 



D. The Two Components 



One of the features of the turnip yellow mosaic virus which has caused it 

 to be of great interest to virus workers is that, unlike the virus preparations 

 which had been obtained previously, this virus always gives rise to two 

 kinds of proteins which appear to be very similar, and which are possibly 

 identical, but only one of which is infectious. The way in which this was 

 discovered is of some interest. The original preparations of the virus were 

 made by the methods outlined above, and on examination by the usual 

 methods were found to behave much as a virus might be expected to. The 

 material was fully crystalline, had a nucleoprotein spectrum, and was highly 

 infectious (2 infections out of 8 at 10-^ gm./ml.). The preparation consisted 

 of uniform spherical particles when examined electron microscopically. The 

 phosphorus content was large, about 2.13-2.24 %, but varied in an inexpHc- 

 able way from preparation to preparation. On examination on the Tiselius 

 apparatus over a range of pH's only one boundary was visible and the 

 isoelectric point was pH 3.75 (Fig. 14). The wet size of the virus was deter- 

 mined by diffusion and the diameter was 280 A. It was with some surprise, 

 therefore, that when it was examined in the ultracentrifuge, it was realized 

 that two compounds were present (Fig. 15). These had sedimentation 

 coefficients of 106 and 49 S; the latter accounted for about a quarter of the 

 total material. For a time it was thought that the 49 S material was the virus 

 and that the lOQ S material represented a 4 : 1 aggregation of the former, but 

 it later proved possible to isolate a few miUigrams of the 49 S ("top") com- 

 ponent by means of an ultracentrifuge partition cell. Fortunately this 

 material was examined by a number of methods. The actual concentration 

 was known to a close approximation from the area of the sedimentation 

 pattern, and this was checked by quantitative serological precipitation. 

 One milligram was set aside for crystallization, and yielded the usual 

 octahedral crystals. One-tenth of a miUigram was used for absorption 

 spectrophotometry, when it was found that instead of having an optical 

 density per cm.^ at 260 m/x of about 0.7, it had a negligible absorption. The 

 rest of the material was used for pentose estimation, which showed that 

 little or no nucleic acid was present, and for infectivity measurements, which 

 indicated that the material was not infectious. 



Later it was found possible to prepare quantities of both components 

 in a fairly pure condition largely by centrifuging. The methods employed 

 are somewhat complex, and are discussed in full elsewhere. The examination 

 of these preparations confirmed that there are, in fact, two substances, a 

 nucleoprotein and a protein, produced by the virus infection. These resemble 

 each other in a remarkable fashion. The points of similarity and of difference 

 are summarized in the following: 



