THE BIOCHEMISTRY OF PLANT VIRUSES 



115 



are the tobacco mosaic viruses (Spooner and Bawden, 1935; Matthews, 

 1949b). 



Being elongated viruses, the potato X viruses show all the usual character- 

 istics of such molecules. The solutions are viscous, show flow birefringence 

 and form two layers on standing. The orientation of the liquid crystalline 

 material is not as perfect as is that of the tobacco mosaic, and Bernal and 

 Fankuchen (1941b) were unable to deduce much about the structure of the 

 particles, except to find that they differed appreciably from the tobacco 

 mosaics, and that the subunit size was not the same. 



XVI. The Use of Nucleic Acid Analogs in the Control of 



Plant Viruses 



When it became evident that the nucleic acid of plant viruses played 

 an important part in their economy, it was soon realized that the possibility 

 existed in modifying the course of the disease by substances which might 

 act as nucleic acid antagonists. 



The earliest attempt of this kind was made by Commoner and Mercer 

 (1952). It had been known for a long time that the tobacco mosaic virus 

 contained uracil as part of the nucleic acid component, and Commoner and 

 Mercer used the readily available analog 2-thiouracil, whose structure is 

 shown in Formula IV, 







H— Ni 



-H 



2 



^/^ \^ / 2-Tliiouracil 



I 

 H 



(IV) 



in an attempt to inhibit nucleic acid synthesis and so to inhibit virus multi- 

 plication in tobacco. In fact thiouracil is exceedingly active against tobacco 

 mosaic virus infection, but only at a concentration which is damaging to 

 the plants. It has, however, been used to prevent spread of the virus in 

 hypersensitive tobacco plants with some success (Holmes, 1955). The con- 

 centrations used are of the order of 10~W, although higher concentrations 

 have been used. Commoner and Mercer showed that the effect of thiouracil 

 could be reversed by uracil. They prepared virus under conditions where 

 inhibition was only partial, and they showed that the virus produced under 



