THE BIOCHEMISTRY OF PLANT VIRUSES 107 



The mild way in which the nucleic acid may be liberated from the virus, 

 by treatment with ethanol of more than 34 % v. /v., in the presence of salt, 

 at neutral pH, and in the cold, is such that it should produce nearly native 

 nucleic acid, provided that enzyme contamination can be prevented. The 

 nucleic acid itself is probably much coiled inside the virus, and probably 

 cross-linked by means of polyvalent bases, but on extraction it forms viscous 

 solutions, probably because it extends when removed from its protein coat. 



X. The Wild Cucumber Mosaic Virus 



The wild cucumber mosaic virus was isolated from a naturally infected 

 wild plant by Freitag (1952) and has been investigated by Sinclair et at. (1957). 

 Its general properties are very interesting, because it resembles in many 

 ways the turnip yellow mosaic virus, with which, however, it does not appear 

 to be related immunologically (P. Kaesberg, personal communication). 



A. Purification 



The virus was grown on Cucurbita pepo (pumpkin) and was purified by 

 alternate cycles of high- and low-speed centrifugation. When examined on the 

 ultracentrifuge, it was found to have two components, which were separated 

 by the use of an ultracentrifuge separation cell, and it was found that the 

 slower sedimenting component was noninfectious. 



Larger-scale separations of the two components were made using the 

 density gradient method of Brakke (1953). Tests of the two zones obtained 

 in a number of runs confirmed that the slower sedimenting component was 

 noninfectious. 



B. The Two Com'ponents 



The faster sedimenting or "bottom" component, which amoimts to about 

 three-fourths of the whole, has a typical nucleoprotein spectrum with a 

 peak at about 260 mjLt. The other or "top" component has a diffuse spectrum 

 with no definite maximum, but it evidently does not contain nucleic acid. 



Ultracentrifugation of the two components at pH 7.0 gave sedimentation 

 coefiicients of 54.4 S and 107.5 S extrapolated to zero concentration. At pH 8, 

 however, only the top component peak was found, presumably because of the 

 loss of the nucleic acid at this pH. This is a most startling observation, and 

 one which should be of the utmost use in further work on this virus. The 

 virus also loses its infectivity when exposed to this pH, as one would expect. 



Electrophoretic examination showed that there was only one component 

 over a fairly wide range of pH's. This is a further indication that the electro- 

 phoretic method is only of use in comparing the surfaces of large molecules. 

 The isoelectric point of the virus is pH 6.6, which is imusually high for 

 viruses. 



