6 VIRUSES 



Batiafies the requirements demanded by bacteriologiatB for acceptance as the 

 causatlre agent of the mosaic disease. 



The ultr«centrifuge can be used to provide further evidence for the iden- 

 tity of the protein and the infectious principle of tobacco mosaic. A special 

 type of ultracentrifuge cell designed by Tiselius, Pedersen, and Svedberg is 

 used. Longitudinal and cross-sectional views are shown in figure 4. 



FIGURE 4 - LONGITUDINAL AND CROSS -SECTIONAL VIEWS OP 

 SEPARATION CELL. (Svedberg and Pedersen "The Ultracentrifuge";. 



The cell consists of the usual plastic block with a sector-shaped slot and two 

 quartz windows. It differs from the ordinary centrifuge cell by having a per- 

 forated barrier about 2/3 of the way from the top to the bottom. The barrier 

 supports a piece of filter paper. After it is filled with virus solution, the 

 cell is placed in the rotor. When the centrifuge is in operation, the particles 

 sediment in the direction indicated as top- to-bottom on the diagram. The virus 

 particles which come into the region of the barrier pass through it. A reason- 

 ably accurate sample of the material remaining above the barrier at the con- 

 clusion of the run can be removed for analysis by chemical or biological means. 

 Jj'rom such analysis one can compute the rate of sedimentation of the biological 

 or chemical entity. In addition, this cell permits the material to be observed 

 by the usual optical methods during the course of the run. 



In an experiment with tobacco mosaic virus, the special centrifuge cell 

 was filled with virus solution and was spun in the ultracentrifuge until the 

 virus protein boundary was about 3/* of the way from the top of the cell to the 

 perforated barrier. Pictures were taken by the usual optical means during the 

 experiment. After the completion of the run, the contents of the top section 

 of the centrifuge cell were removed. Prom the optical data on the sedimenting 

 boundary, the sedimentation constant of the nucleoprotein was calculated. Also 

 from these data, the position of the boundary at the time the centrifuge stopped 

 was determined. From the position of the boundary with respect to that of the 

 barrier at the end of the experiment, it was a simple problem in solid geometry 

 to calculate the fraction of the protein which moved out of the top compartment 

 during the course of the sedimentation. 



If the infectious principle sediments exactly as fast as the protein, the 

 ratio of the Infectlvity of the contents of the top compartment to that of the 

 original virus solution should be the same as the ratio of protein concentrat- 

 ions. Infectlvity titrations were carried out both on the contents of the top 

 compartment and on the original by the method of Price and Spencer, i'he results 

 are presented in Table I. 



