THE BIOCHEMISTRY OF PLANT VIRUSES 105 



subunits as one might expect from the studies of Klug et al. (1957), but the 

 resolution required is at the limit of present-day techniques (Fig. 18). As 

 an alternative to the snub dodecahedron a model based upon the icosahedron 

 would have rather similar properties. It is also quite likely that the 60-A 

 subunit is itself composed of a number (probably 2) of subunits. 



Fig. 17. Sketch of snub dodecahedron made up from 60 spherical subunits. This 

 structure is proposed for the turnip yellow mosaic virus by KJug et al. (1957). 



E. Amino Acid Composition 



The amino acid composition was first investigated by paper chromato- 

 graphic analysis (Markham et al., 1948a), but the first quantitative analyses 

 were made by Roberts and Ramasarma (1952) who employed microbiological 

 assay methods. More recently Eraser and Cosentino (1957) have analyzed 

 the protein component of the virus and of the top component by ion-exchange 

 chromatography. The results of these analyses are tabulated in Table IV. 



There are a number of discrepancies between these sets of data, the most 

 striking one being the apparent absence of cysteine in the analyses made by 

 Fraser and Cosentino. Analyses made in Cambridge indicate that cysteine is 

 indeed present as found by Roberts and Ramasarma. 



The tryptophan analyses made by the latter are also of interest, because 

 they indicate a subunit size of about 20,000, a value which is consistent with 

 analyses made on two strains of the virus having vastly different acid 

 compositions and so far unpublished. The fine structure of the protein, 

 which would have some 280 amino acids, is also under investigation. 



An interesting consequence of these analyses is that the number of basic 

 amino acids present in the virus is such that they cannot neutralize more than 

 a fraction of the phosphorus atoms in the nucleic acid, so that the proportion 

 of base present in the virus must be quite high. The nature of the basic 

 substances present is also being investigated. 



