THE BIOCHEMISTRY OF PLANT VIRUSES 51 



method is suitable for the stepwise degradation of polypeptides, and it may 

 in fact be applied to very small amounts of material. 



Carbox3rpej)tidase may also be used for the sequential splitting-off of 

 amino acids, this time from the C-terminal end. Unfortunately, the rate at 

 which the individual amino acids are split oif depends, to a large extent, 

 on the nature of the amino acid next in sequence in the chain, so that it is 

 necessary to determine the rates at which the successive amino acids are 

 released; even then it may not be possible to identify more than the first one 

 or two in each sequence. In the case mentioned earlier of the intact tobacco 

 mosaic virus, only the amino acid, threonine, is liberated, because the 

 succeeding residues are alanine followed by proline, and the prolyl peptide 

 bond is completely resistant to attack by this enzyme. 



Finally, having determined the sequences of a large number of peptides, 

 the polyjDeptide sequence may then be determined provided that enough 

 overlap of sequence exists to enable the pieces to be fitted together, like a 

 jigsaw puzzle, in an unambiguous way. Needless to say this has not yet been 

 accomphshed for any virus. 



IV. The Nucleic Acids 



The nucleic acids were first recognized in animal cell nuclei in the latter 

 part of the .last century by Miescher (1871) who obtained a phosphorus-rich 

 compound, which was evidently acidic, and which he realized was one 

 of the important organic phosphorus compounds of animal tissues. Sub- 

 stances of this general tyjDC were later isolated from plant cells (Altmann, 

 1889; Osborne and Harris, 1902), but the general impression of the early 

 workers was that animal tissues contained one type of nucleic acid, and plant 

 tissues another. There are, indeed, two main types of nucleic acid, which 

 differ mainly in their sugar constituents, but most cells of animals and plants 

 contain both. Viruses usually contain one or the other, and plant viruses, 

 as far as is known at present, contain only that nucleic acid which has ribose 

 as its sugar, or ribonucleic acid, as it is now called. Strictly speaking, ribo- 

 nucleic acid is a term covering a family of related substances, all having 

 similar structures but differing in very subtle ways. These differences are 

 thought to be related to the biological function of the nucleic acids. 



For very many years, the nucleic acids were regarded as relatively simple 

 compoimds having molecular weights of the order of 1200; as they were 

 invariably associated with protein, they were regarded rather as an adjmict 

 to proteins, in combination ^vith which they formed the nucleoproteins, 

 than as distinct chemical substances possibly having biological activity of 

 a high order. Indeed the term "prosthetic group" was originally used to 

 denote the relationship between nucleic acid and protein by Kossel (1893). 



