34. THE RIBONUCLEIC ACIDS OF VIRUSES 291 



that of protein free TMV-RNA alone. The possibility of a bonding between 

 RNA and protein through triester linkages of RNA phosphoric acid with 

 protein amino acids is ruled out by the experiments of Koshland et a/. 145 

 who separated the protein coat of TMV by the action of detergent in the 

 presence of H 2 18 0. If such bonds were present, H 2 18 should be found in 

 RNA phosphate after isolating the RNA by detergent treatment. However 

 such was not the case. The most probable type of binding between RXA 

 and protein may be a type of salt binding between the acid phosphate 

 groups of RXA and the basic groups of amino acids. 



2. The Ribonucleic Acids of Other Plant Viruses 



Kaper and Steere 146 have isolated an infectious nucleic acid from a 

 spherical plant virus, tobacco ringspot virus (TRSV) by a modification of 

 the heat-denaturation method. One volume of 0.5-1 % virus solution was 

 quickly added to 2 volumes of hot (95-100°C.) M NaCl solution, heating 

 was continued for exactly 35 seconds, during which time the mixture was 

 agitated continuously. After cooling the solution, the virus protein was 

 separated from the nucleic acid by centrifugation and the nucleic acid 

 precipitated afterward by ethanol. The yield of RXA varied between GO- 

 TO % of the nucleic acid in the virus. The infectivity of these preparations 

 with a protein content not exceeding 0.7 % ranged from 0.1-1.0% as com- 

 pared with the nucleic acid in the virus. Some preparations were also made 

 with the phenol method, but the yield was usually very low and the in- 

 fectivity not higher than of those prepared by heat denaturation. 



The usual tests (incubation with ribonuclease, ultracentrifugation, 

 storage for several days in the cold) conducted with this nucleic acid have 

 given evidence that the associated infectivity cannot be due to contamina- 

 tion with residual whole virus. Also, in contrast to the whole virus, the 

 infectivity of the nucleic acid was influenced by the concentration of phos- 

 phate buffer present in the medium used for inoculation; the average lesion 

 count increased gradually up to 0.2 M buffer concentration. The molecular 

 weight of the nucleic acid preparation was not determined. Twenty per cent 

 less phosphorus and nitrogen than expected on the basis of the nucleotide 

 composition of the nucleic acid was found, suggesting the presence of a 

 nonnitrogenous impurity. 



Up to date, RXA has been isolated from plant viruses other than TMV 

 and TRSV only by methods which do not completely exclude the degrada- 

 tion of the RXA. For example, Dorner and Knight 147 have studied the RXA 



145 D. E. Koshland, Jr., X. S. Simmons, and J. D. Watson, J. Am. Chem. Soc. 80, 105 

 (1958). 



146 J. M. Kaper and R. L. Steere, Virology 7, 127 (1959). 



147 R. W. Dorner and C. A. Knight, J. Biol. Chem. 205, 959 (1953). 



