VARIATION AND ITS CHEMICAL COREELATES 145 



In between the two ends of each subunit peptide chain there are in the 

 neighbourhood of 160 amino acid residues. To determine the sequence of 

 these is a formidable task. However, methods are available which permit a 

 rather rapid survey of the chain by spUtting it into a series of peptide frag- 

 ments. For example, treatment of the TMV protein with trypsin yields about 

 13 peptides, which can be separated by a two-dimensional paper electro- 

 phoresis-paper chromatography procedure (Knight, 1957). By treatment of 

 the paper with a suitable reagent that develops a color by reaction with the 

 peptides, a characteristic map of the peptides is obtained. This technique 

 has been applied to only a few strains of TMV, with the following results: 

 The peptides of TMV and M appear to be so similar that it has not been 

 possible as yet to distinguish between their maps. On the other hand, the 

 peptide maps of TMV and HR are widely different, as might be anticipated 

 from the very different amino acid compositions of the two strains. The 

 peptides of YA appear to coincide in main with those of TMV, but differ in 

 three or four instances. Isolation of quantities of peptides by such procedures 

 as countercurrent distribution and ion exchange column chromatography is 

 making possible the determination of amino acid sequences of individual 

 peptides; eventually these will be put together to develop complete sequences 

 for the various strain proteins. 



b. Structural Features of Plant Viruses Not in the TMV Group. Little has 

 been done as yet on the structures of other plant virus strains. However, 

 bushy stunt virus, potato virus X, and cucumber viruses 3 and 4 have been 

 found, Uke TMV, not to have free iV-terminal residues while having character- 

 istic kinds and numbers of C-terminal groups (Niu et al., 1958). The absence 

 of free A^-terminal groups in these viruses appears to be explained, as in the 

 case of TMV, by the presence of iV-acetyl, but each virus has been found to 

 have a different and characteristic acetyl-iV-terminal amino acid sequence 

 (Narita, personal communication). 



c. Summary of Structural Features of the Proteins of Virus Strains. There 

 are as yet too few data to generahze extensively on the essential structural 

 features of plant virus strains. Nevertheless, the picture seems so clear with 

 the TMV strains that the following theory can be proposed for further 

 testing as new information becomes available. It is assumed that the protein 

 moieties of all macromolecular plant viruses are made up of subunits (Crick 

 and Watson, 1957). There is evidence to support this assumption in every 

 case so far examined (TMV, BSV, PXV, CV3, and CV4). Each variant strain 

 of a group of viruses will have the same number of subunits, which will 

 be indicated by the presence of equal numbers of terminal amino acid 

 residues. These terminal residues will not only be equal in number but also 

 in kind. 



VOL. II — 10 



