VARIATION AND ITS CHEMICAL CORRELATES 139 



case for strain relationship. It will be recognized, of course, that none of the 

 tests is infaUible. All of them have limitations and these limitations need 

 especially to be considered in borderline cases. 



III. Chemistry of Virus Strains 



The plant viruses isolated and purified thus far have proved to be solely 

 nucleoproteins. The chemistry of these viruses is therefore conveniently 

 resolved into studies of protein and nucleic acid components. 



A. Protein Components 



1. Composition of Strain Proteins 



The common amino acids found in most proteins are also found in viruses. 

 No unusual amino acids, D-isomers, or extraordinary amounts of individual 

 amino acids have been observed so far. Thus, there is nothing unique about 

 viral protein compositions which clearly distinguishes them from other bio- 

 logically active agents. However, individual viruses, and often strains of the 

 same virus, have been found to possess distinct and characteristic protein 

 components. 



The tools for complete analysis of proteins for their amino acid content 

 have been available since only about 1945. First, microbiological assay and 

 then ion exchange chromatography have been applied to the products of 

 acid and alkahne hydrolysis of the viruses. The agreement of results obtained 

 by the two methods has been surprisingly good in the few instances in which 

 appropriate comparisons have been made. 



In Table II are given the results of some amino acid analyses of a few plant 

 viruses. These illustrate the differences between unrelated viruses and provide 

 a basis for comparison with the results obtained with different strains. The 

 values reported for TMV by Ramachandran (1958) and shown in column 2 

 are probably the most accurate now available for this widely studied virus. 



a. Strains of Tobacco Mosaic Virus. The variant strains of T]MV have been 

 more widely studied than strains of any other virus for several reasons. 

 Historically these were among the first to be recognized as variants of a 

 plant virus; subsequent studies have shown them to be very numerous [over 

 400 reported (Kunkel, 1947)], generally easy to isolate and to purify, readily 

 transmitted, and characterized by distinctive quahtative and quantitative 

 symptoms in appropriate easily grown hosts. 



Physical-chemical studies have been made on various strains of TMV in 

 laboratories in different parts of the world. However, certain selected strains 

 famihar to the author will be used primarily to develop the picture of strain 

 chemistry, not because these strains are more important than others, but 



