34. THE RIBONUCLEIC ACIDS OF VIRUSES 249 



may be concluded that the nucleic acids of different strains possess certain oligo- 

 nucleotides of identical composition. The identical composition of the different 

 ribonuclease-resistant cores does not necessarily imply a structural identity, mean- 

 ing identical nucleotide sequences. Residues of identical composition may have quite 

 different nucleotide sequences. Further, the linkage of these oligonucleotides with 

 each other in the original nucleic acid could be quite different in the different species 

 of RNA; some may be joined together by uracil or cytosine mononucleotides and 

 others by smaller pyrimidine oligonucleotides. Thus, Reddi 20 observed that the 

 amount of pyrimidine mononucleotides released upon ribonuclease digestion was 

 approximately identical for 3 strains of TMV, whereas the amount released by a 

 fourth strain was significantly larger by some 40%. It may be concluded that the 

 nucleic acid of the latter strain (strain M) can be distinguished from the other 3 

 strains with respect to the intramolecular distribution of pyrimidine mononucleo- 

 tides. 



The mononucleotide arrangement in the nucleic acids of some TMV strains must 

 be something more complex than the mere repetition of one simple pattern involving 

 only a few residues as was made clear by comparing the numbers and types of split 

 products obtained by Reddi and Knight 19 with those expected from the degradation 

 of a hypothetical "random" RNA under the same conditions. This hypothetical RNA 

 has the same relative number of adenylic, cytidylic, guanylic, and uridylic residues 

 as do the RNA's of the viral strains studied. The sequence of these residues along 

 the hypothetical nucleotide polymer is assumed to be completely random. Hart 21 

 calculated that the core composition for the viral nucleic acid digest did not differ 

 from that expected for a random polymer. 



In contrast to the studies which have been discussed so far, Commoner and Basler 22 

 found different RNA contents and different base ratios for more than 50 separate 

 preparations of one strain of TMV. These authors claimed that the values were in- 

 fluenced by the duration of infection and the nature of the tissue from which the 

 virus was isolated. However, the analytical method could not completely exclude 

 impurities, such as foreign protein and a maximum of 5% nonviral RNA. Since RNA 

 from tobacco leaves has a markedly different adenine and uridine content than TMV- 

 RNA, 23 a small amount of cellular RNA could be responsible for the reported vari- 

 able base ratios of TMV-RNA, 



2. Animal Viruses 



There is also a variable percentage of RXA in those animal viruses that 

 contain the ribose type of nucleic acid. Several typical RNA animal viruses 

 are listed in Table III. A simple calculation shows that the absolute amount 

 of RNA in all these viruses, with one exception, corresponds to a "molecu- 

 lar weight" of 2 X 10 6 . 10 The exception is Newcastle disease virus which 

 also seems to have a much higher particle weight than the other viruses 

 listed in Table III. On the other hand, the purity of the Newcastle disease 

 virus preparations which have been analyzed is questionable."' 3 



20 K. K. Reddi, Biochim. et Biophys. Acta 25, 528 (1957). 



21 R. G. Hart, Proc. Natl. Acad. Sci. U.S. 43, 457 (1957). 



22 B. Commoner and E. Basler, Virology 2, 477 (1956). 



23 K. K. Reddi, Biochim. et Biophys. Acta 23, 208 (1957). 



23a W. Schafer, in "The Viruses" (F. M. Burnet and W. M. Stanley, eds.), Vol. I, p. 

 475. Academic Press, New Vork, 1959. 



