THE BIOCHEMISTRY OF PLANT VIRUSES 87 



The physical properties of the nucleic acid were further investigated by 

 Cohen and Stanley (1942), who prepared their material by the heat denatura- 

 tion method, and they concluded that the nucleic acid was vastly larger 

 (a molecular weight of 300,000) than yeast nucleic acid, which at the time 

 was thought to have a molecular weight of about 1200. They also fomid that 

 the virus nucleic acid was relatively easily degraded to smaller molecules. 



When chromatographic methods for the analysis of the nucleic acids v/ere 

 developed, the composition of the nucleic acid of various strains of tobacco 

 mosaic virus was reinvestigated. Using paper chromatography, Markham 

 and J. D. Smith (1950) showed that the sugar in the nucleic acid was, as 

 expected, ribose, a finding later confirmed again by the same method, and 

 by the isolation of ribose as the di-w-propyl mercaptal by MacDonald and 

 Knight (1953). The proportions of adenine, guanine, cytosine, and uracil 

 were foimd to be 1.17 : 1.05 : 0.71 : 1.06 on a molar basis; this has been con- 

 firmed by a number of workers. (Knight, 1952; Schramm and Kerekjarto, 

 1952). It was also found that the base ratios of a number of strains of this 

 virus were very similar, if not identical. The actual structure of the nucleic 

 acids must, of course, differ in some way, but this difference may be reflected 

 in a very minor way in the actual proportions of the bases. (Markham and 

 J. D. Smith, 1950; Reddi and Knight, 1956). The only information as to the 

 arrangement of the bases has been supplied by Reddi (1957b), who found that 

 the nucleic acid of the "masked" strain of the tobacco mosaic virus had many 

 more pyrimidine residues which could be released as mononucleotides by 

 pancreatic ribonuclease, than had the other strains wliich he investigated. 

 If this result is not due to a chance contamination of the virus nucleic acid 

 with another enzyme, his findings would indicate a very large difference in 

 the nucleotide sequence in the masked strain of the virus. 



The apparent constancy of the composition of tobacco mosaic virus 

 nucleic acid has been questioned by Commoner and Easier (1956) who have 

 obtained results which suggest that there are vast variations in the base 

 ratios at different times after the plants were infected with the virus. The 

 significance of these claims is in some doubt, because the methods by which 

 the results were obtained were novel and apparently had not been developed 

 adequately. There is also a certain amomit of internal evidence which 

 suggests that the method of hydrolysis used was not sufficiently quantitative 

 to make the results particularly meaningful. It would also appear likely 

 that other workers would have noticed fairly large fluctuations in their 

 analytical values, and there is every indication that this is not so. 



The nucleic acid of this virus, like that of the turnip yellow mosaic virus, 

 is evidently synthesized through compounds having a nucleoside 5'-phosphate 

 structure (possibly the pyrophosphate or triphosphate) because phosphorus 

 labeling of the 5'-nucleotides proceeds unevenly when virus is grown in the 



