THE BIOCHEMISTRY OF PLANT VIRUSES 113 



particles, as seen by themselves on electron microscopy, are essentially 

 spherical or, more probably, polygonal (Kaesberg, 1956). 



C. Physical Properties 



The virus, like many other spherical viruses, is hydrated in solution, 

 having a diameter of 286 A (Leonard et al., 1953). The partial specific 

 volume is 0.700 and the molecular weight is 6.6 millions (Lauffer et al., 1952). 

 The isoelectric point of the type strain and of a yeUow mutant derived from 

 it is pH 5.9 (MacDonald et al, 1949). 



The virus is stable over a fairly narrow pH range, this being from pH 4 to 8. 



D. Chemical Composition 



The virus contains about 21 % of nucleic acid, which has been shown by 

 Dorner and Knight (1953) to be ribonucleic acid. The base composition is: 

 adenine, 1.03; guanine, 1.04; cytosine, 0.92; and uracil, 1.01, on a molar basis. 

 This is probably the closest to unity in ratios yet found. As with tomato 

 bushy stunt \'irus the nucleic acid is not released when the virus is denatured 

 by boiling. 



XV. Potato Virus X (Potato Latent Mosaic Virus) 



The potato X virus is one of the more important viruses in the world. 

 It affects a large percentage of all potatoes grown, and even in its mildest 

 form may cause 10 % or more reduction in the potato crop (K. M. Smith and 

 Markham, 1945). The potential loss of food in the whole world, where about 

 2.5 X 10^ tons of potatoes are grown, is staggering. Allowing 10 % infection 

 with this virus, which is almost certainly an underestimate, the loss is 

 probably over 2 million tons of food every year. The amount of attention 

 paid to this virus is hardly in keeping with its economic importance. 



The potato X virus is of some interest because it was probably one of the 

 first viruses to be recognized as being mainly protein in nature (Bawden and 

 Pirie, 1936), and it is unusual in that it is readily digested by trypsin. 



Although primarily a pathogen of the potato, in which it spreads by plant- 

 to-plant contact, the potato X virus will grow readily on a number of plants, 

 and for experimental work is usually grown on tobacco (Fig. 21). This 

 transfer of hosts results in a gross change in the ability of the virus to infect 

 potatoes, a change which might be accompanied by chemical differences. 

 An even more dramatic change is shown on inoculation of the virus to 

 Cyphomandra betacea (Matthews, 1949a), when the severity of the disease 

 produced on transfer back to tobacco is greatly increased. 



VOL. II — 8 



