34. THE RIBONUCLEIC ACIDS OF VIRUSES 251 



pared the ratio adenine + uracil/guanine + cytosine for RNA 24 from A 

 and B influenza. The average values were 1.25 for type A and 1.40 for type 

 B. This ratio is 0.6 for the RNA in an alkaline digest of uninfected host 

 cells. This would mean that even a minute contamination of the virus 

 preparation with host cell RNA would result in a significant alteration of 

 the base ratio of viral RNA. The base ratios of various strains of one 

 serotype are, however, approximately the same in all cases studied so far. 

 In summary : there are no qualitative differences between viral and host 

 cell RNA. Rare bases, such as the methylated bases found in cellular RNA, 25 

 have not been observed in viral RNA. Considering the accuracy of the 

 analytical methods used, it seems safe to state that no significant differences 

 have been found, to date, in the base compositions of RNA from related 

 virus strains. In contrast, large differences have been found in the base 

 ratios of RNA from different types of viruses. The fact that the absolute 

 amount of RNA contained in a single virus particle corresponds to a "mo- 

 lecular weight" of 2 X 10 6 or perhaps a multiple of this value (in the case 

 of Newcastle disease virus, for example) suggests that a certain minimal 

 "amount" of RNA is necessary in order to be biologically active. 



III. The Ribonucleic Acids of Plant Viruses 



Since it is much easier to purify plant viruses than animal viruses and 

 since the virus yield of the former is much greater than the latter, plant 

 viruses present certain advantages as objects for chemical studies. For 

 example, a liter of sap from infected plants can contain up to 2 gm. of virus, 26 

 whereas the amount of purified poliovirus which can be harvested from a 

 liter of infected tissue culture fluid is less than 1 mg. 27 Thus, it is clear that 

 it is quite possible to obtain larger amounts of virus RNA from plant 

 viruses. 



TMV is the virus of choice for most studies on plant viruses. It was the 

 first virus to be obtained in pure form as paracrystalline needles, 28 and 

 further, in comparison to other plant viruses, it is relatively simple to ob- 

 tain in rather large quantities. Using TMV, it was also possible, for the 

 first time, to separate the two components, RNA and protein, in such a 

 way that the isolated RNA was still biologically active. This observation 

 led to a series of studies on the structure, physical, chemical, and biological 



^ I). Elson and E. Chargaff, Nature 173, 1037 (1954). 



25 J. W. Littlefield and D. B. Dunn, Biochem. J. 70, 642 (1958). 



26 G. Schramm, in "Die Biochemie der Viren" (H. Bredereck and E. Midler, eds.), 

 p. 135. Springer, Berlin, 1954. 



27 C. E. Schwerdt and F. L. Schaffer, Virology 2, 665 (1958). 



28 W. M. Stanley, Science 81, 644 (1935). 



