630 PLANT GROWTH AND PLANT COMMUNITIES 



Somewhat different results were obtained with tobacco-mosaic 

 virus by Commoner and Rodenberg ( 1955 ) , and by Delwiche et al. 

 (1955), using N^^. In both cases the nucleic acid-free protein and 

 nucleoprotein had similar contents of isotopic ammonium. Commoner 

 and Rodenberg suggested that the nucleic acid-free protein and virus 

 protein are synthesized at the same time and from the same nitrog- 

 enous (non-protein) source. While Delwiche et al. related the non- 

 virus protein to virus synthesis, they did not consider it to be a direct 

 precursor of virus. 



Electron microscopy and chemical studies have further related 

 the anomalous proteins to virus synthesis ( Markham, 1951; Consentino 

 et al., 1956; Fraser and Consentino, 1957; Newmark and Fraser, 1956 ) . 

 The "top" component of turnip yellow-mosaic virus has the same gross 

 morphology and exactly the same amino-acid composition as the pro- 

 tein shell of the virus particle. The amino-acid compositions of the 

 anomalous protein and of the nucleoprotein of tobacco-mosaic virus 

 have also been sliown to be identical. The non-infectious protein of 

 tobacco-mosaic virus can be combined with infectious nucleic acid to 

 produce an infectious nucleoprotein ( Takahashi, 1959a ) . The closeness, 

 in many respects, of the anomalous proteins and virus proteins would 

 indicate that they are all part of virus synthesis. Takahashi (1959b) be- 

 lieves that the protein accompanying tobacco-mosaic-virus synthesis is 

 indeed the protein used in the production of virus nucleoprotein; that 

 protein and nucleic acid are synthesized separately and the final step 

 in virus production is an assembly of the two components. 



Nucleic acid. Fewer comparative studies have been made of 

 healthy and virus-infected tissue in regard to nucleic-acid composition. 

 Prior to the detection of tobacco-mosaic virus in inoculated plants, the 

 nucleic acid in infected tissue exceeds that in healthy tissue by an 

 amount slightly in excess of the nucleic acid incorporated into the 

 virus produced (Easier and Commoner, 1956). In other words, there 

 is an accumulation of nucleic acid slightly in excess of that which will 

 go into virus particles. As virus appears, this excess rapidly diminshes, 

 and an actual deficiency develops. When the excess is analyzed in 

 terms of uracil, cytosine, and adenine, the concentrations of these 

 almost equal the respective amounts going into virus. Therefore Easier 

 and Commoner reasoned that the excess nucleic acid prior to virus ap- 

 pearance represents a nucleic-acid precursor to the virus nucleic acid. 

 Carbohydrates. Much attention has been given to the effects of 

 virus infection on carbohydrate and nitrogen metabolism and to the 

 C/N ratio in plants. Dunlap (1930), after studying the literature and 

 experimenting with mosaic and yellows diseases of a number of plants, 

 suggested that virus diseases might be grouped according to their 

 effect on the C/N ratio. Mosaic diseases were found to be accompanied 



