THE BIOCHEMISTRY OF PLANT VIRUSES 79 



leave lysine or arginine at th.e.carboxyl end. This is a trick calculated to 

 facilitate the separation, wliich was carried out on a sulfonic acid (Dowex 50) 

 ion exchange resin. Peptides containing no basic group are not retained by 

 such a resin at all strongly, and in this way Narita (1958b) was able to isolate 

 adequate amounts of two peptides, one from each enzymatic digest. These 

 peptides, had no N-terminal amino acid residues, but were both fomid to be 

 acylated. The acyl group was isolated as the hydrazide and found to be an 

 acetyl group. The peptide isolated from each digest contained both serine 

 and tryosine only, in approximately equivalent amounts, with tyrosme as 

 the C-terminal end. There are no quantitative data which would show that 

 there is one such acetyl peptide per chain, and, as Narita has remarked, it 

 is possible that this peptide may come from the blocked e-amino group of 

 lysine. It does, however, seem quite likely that the peptide really represents 

 the end of the peptide chain, which is probably iV-acetyl, seryl, tyrosyl-. 

 The possibility of the iV-acetyl serine arising from 0-acetyl serine by N-^0 

 migration seems to have been resolved satisfactorily by the isolation of the 

 same peptide under both acid and neutral conditions, but this migration 

 might take place at an early stage in the development of the virus. Whichever 

 position was originally occupied by the acetyl group, however, it is certain 

 to cause complications in the theory of the "coding" mechanism involved 

 in protein synthesis. Neither acetic acid nor iV-acetyl serine is an amino acid, 

 while 0-acetyl serine would be a new amino acid. 



It is interesting to recall here that the presence of acetyl groups was 

 discovered in tobacco mosaic virus 17 years before the acetyl serine residue 

 was recognized. 



F. The Position of the Nucleic Acid 



It is now clear that the main function of the protein component of small 

 viruses is to protect the nucleic acid part. It became evident some time ago 

 that several viruses had their nucleic acid located below the surface (Markham, 

 1951; Hershey and Chase, 1952; Watson, 1954). Since this time several 

 workers have shown that the nucleic acid of the tobacco mosaic virus 

 occupies an axial position. 



The position of the nucleic acid may be revealed by a number of methods. 

 Stahmaim and Kaesberg (1955) were the first persons to detect it as an axial 

 thread in virus which had been freeze-dried (Rice et al., 1953), and shortly 

 after this Hart (1955a,b) found that it could be demonstrated by the action of 

 the anionic detergent, dodecyl sulfate, on the virus in solution. Dodecyl sulfate 

 had been found by Sreenivasaya and Pirie (1938) to split off the protein 

 of the virus, and it has since been used as a convenient method for the pre- 

 paration of the virus nucleic acid. Dodecyl sulfate probably competes with 



