VIRUSES AS MOLECUIiES 



yiGUHE 6 - LEAVES OP TOBACCO PLANTS DISEASED V/ITH VARIOUS 

 STRAINS OF TOBACCO L.OSAIG VIRUS. 'illV-normal , YA-yellow aucuba, 

 GA-green aucuba, M-Holmes 'masked, J14D1- a derived mutant, and 

 HR-ribgrass. (W. K. Stanley and C. A. Knight, Cold Spring 

 Harbor Symposia on Quantitative Biology 9, 22^ {194-1; }. 



It differs from ordinary tobacco mosaic in that it produces a systemic infect- 

 ion in ribgrass, that is, an infection which spreads throughout the plant. It 

 is believed that this virus is a strain of tobacco mosaic virus, because, if 

 a tobacco plant is first infected with tobacco mosaic virus, it cannot be sub- 

 sequently infected with ribgrass virus. 



A nucleoprotein was isolated from tobacco plants Infected with the rib- 

 grass virus strain. When examined with the electron microscope, this virus 

 was seen to be made up of rod-shaped particles very much like those of tobacco 

 mosaic virus. However, when examined by other means, differences were observed. 

 In the first place, the ribgrass virus was found to have a lower electro- 

 phoretic mobility than the tobacco mosaic virus . It is well known that if a 

 protein is electrolyzed in a U tube, it will move towards the positive or the 

 negative electrode with a characteristic speed for given conditions. Tobacco 

 mosaic virus migrates toward the positive electrode faster than ribgrass virus 

 under certain fixed conditions. This means that there must be some difference 

 in the chemistry of the surface of the two viruses. However, even a more pro- 

 found difference between the two virus proteins was demonstrated by Knight of 

 the Rockefeller Institute. By careful chemical analysis he found that the 

 amino acid composition of ribgrass virus differed from that of tobacco mosaic 

 virus. The results are shown in Table II. 



