VARIATION AND ITS CHEMICAL CORRELATES 149 



strains of TMV, BSV, and probably of PXV have been found in each case to 

 have nucleic acids of apparently identical composition. Hence, it has been 

 suggested that identity of compositions of the nucleic acids of two viruses 

 can be taken as evidence for strain relationship. 



2. Structural Features of Strain Nucleic Acids 



If, as present data suggest, strains of plant viruses do contain nucleic acids 

 of the same composition, these nucleic acids could conceivably be different 

 in function if the nucleotides were arranged in different sequences. A diffi- 

 culty hes in the current lack of a systematic procedure for determining the 

 sequence of nucleotides in polynucleotide chains. However, a method for 

 probing possible structural differences is provided by the use of the enzyme, 

 ribonuclease. Pancreatic ribonuclease has been shown to be a highly specific 

 phosphodiesterase, which will hydrolyze secondary phosphate esters of 

 pyrimidine riboside 3'-phosphates, but not of purine riboside phosphates. 

 Two nucleic acids of the same composition, but having different sequences 

 of purine and pyrimidine nucleotides, should therefore yield different pro- 

 ducts when digested with pancreatic ril onuclease. 



a. Strains of TMV. As a beginning, the composition was determined of the 

 residual (trichloroacetic acid-precipitable) material after ribonuclease diges- 

 tion of the nucleic acids of each of 5 strains of TMV (Reddi and Knight, 

 1956). However, withia the hmits of the method of analysis used, the acid- 

 insoluble ribonuclease-resistant residues of all 5 strains were found to have 

 the same composition. 



Turning to the acid-soluble fractions of the ribonuclease digests of strain 

 nucleic acids, quantitative estimations were made of the mononucleotides 

 present (Reddi, 1957). Only the pyrimidine nucleotides, cytidyhc anduridyhc 

 acids were found. This was to be expected from the known specificity of 

 pancreatic ribonuclease. As shown in Table VI, the amounts of cytidyhc and 



TABLE VI 



Amounts of Cytidylic and Ueidylio Acids Released from Nucleic Acids of 

 Some Steatns of TMV by 12-Hour Digestion with Pancbeatic Ribonuclease at 



23°C. AND AT pH 7.6« 



„ . ]VIicrograms cytidylic acid/mg. Micrograms uridylic acid 



nucleic acid /mg. nucleic acid 



128 

 121 

 124 

 178 



<» Adapted from Reddi, 1957. 



