VARIATION AND ITS CHEMICAL CORRELATES 147 



The three major constituents of nucleic acids are phosphoric acid, a pentose 

 sugar, and the bases — i.e., purines and pyrimidines. The first two are not 

 particularly distinctive, although it could be quite significant were the pen- 

 tose other than ribose. Ribose has been positively identified as the sugar in 

 TMV and some of its strains (Knight, 1954), and it has been shown by 

 chromatographic methods to be the purine-bound sugar in strains of BSV 

 (de Fremery and Knight, 1955) and in turnip yellow mosaic virus (Markham. 

 and Smith, 1951). However, in most plant viruses, the presence of ribo- 

 nucleic acid and the absence of deoxyribonucleic acid have been assumed on 

 the basis of results of colorimetric reactions distinguishing between pentose 

 and deoxypentose. While at present there is little reason to suspect that plant 

 viruses contain sugars other than ribose, it must be remembered that this has 

 been rigorously demonstrated in very few cases. 



The contents of individual purine and pyrimidine bases have been deter- 

 mined for the nucleic acids of several plant viruses and are summarized in 

 Table V. As is shown in the table, and as was indicated by the earliest 

 analyses of this sort by Markham and Smith (1951), the nucleic acids of 

 different viruses contain demonstrably different proportions of the purine 

 and pyrimidine bases and hence of the nucleotides containing these bases. 

 In contrast, 15 strains of TMV were found to have nucleic acids so similar in 

 composition that they could not be distinguished from one another (Knight, 

 1952; Black and Knight, 1953). These were the strains whose symptoms are 

 illustrated in Fig. 2. Similar results were obtained on 2 strains of TMV by 

 Cooper and Loring (1954). Actually, the earher analyses of Markham and 

 Smith (1950) indicate a great similarity among strains of TMV, except that 

 their analyses on the type strain appear now to have been in error, and CV4 

 was included as a strain of TMV, whereas there is now some doubt that it 

 belongs in the TMV group, at least in the sense that the above 15 strains do. 

 Since the results of the analyses on different strains agree as closely as 

 separate analyses on the same strain, the results have been averaged for 

 Table V. (Certain corrections have been apphed, as indicated in the footnote 

 to the table, to give, it is beheved, as accurate an estimate as present data 

 permit.) 



The nucleic acids of the 3 strains of tomato bushy stunt virus, whose 

 symptoms are illustrated in Fig. 4, were also analyzed for their content of the 

 purine and pyrimidine bases usually found in ribonucleic acids. These strains 

 of BSV, hke those of TMV, showed no significant differences in nucleic acid 

 composition. Hence, an average figure for the three strains is given in 

 Table V. 



Markham (1953) has analyzed the nucleic acids of 7 strains of potato virus 

 X and reports them to be very similar in composition, although the actual 

 analytical data are not presented. 



