$2 AN INTRODUCTION TO THE STUDY OF VIRUSES 
acid 13:5 per cent, glutamic acid 11-3 per cent, which accounts for 
the acidity of the protein, cysteine (no cystine) 0-69 per cent, glycine 
1-9 per cent, isoleucine 6-6 per cent, leucine 9°3 per cent, phenylalanine 
8-4 per cent, proline 5:8 per cent, serine 7:2 per cent, threonine 9-9 
per cent, tryptophane 2-1 per cent, tyrosine 3°8 per cent, valine 
9*2 per cent. 
There is no histidine and no methionine; on the other hand, a 
distantly related strain of tobacco mosaic virus, obtained from plantain, 
contains 0°72 per cent histidine and 2-2 per cent methionine, both of 
which are absent in the type virus. 
Two other distantly related viruses, known as cucumber viruses 
3 and 4, contain very little glutamic acid and no sulphur-containing 
amino acids. It is also possible to differentiate between the type 
virus, the plantain virus and cucumber viruses 3 and 4 on their aromatic 
amino acids, irrespective of the host plant from which they are taken. 
(Table II.) 3 
TABLE II 
AROMATIC AMINO ACIDS AND PHOSPHORUS IN STRAINS 
OF TOBACCO MOSAIC VIRUS AND IN 
CUCUMBER VIRUSES 3 AND 4 





Virus | Tyrosine Tryptophane | Phenylalanine | Phosphorus 
per cent | per cent per cent per cent 
Tobacco mosaic . 3°8 4°5 6:0 0°56 
Yellow aucuba , 3°9 4:2 63 0°52 
Green aucuba ‘ 3°9 4:2 61 0°54 
Plantain strain é 6:4 a5 4°3 0°53 
Masked strain : 3°9 4°3 6:1 0°54 bie 
(14.6 } d 3°8 4°4 6-1 0°55 
Cucumber virus 4 . 3°8 I*4 102 0°54 
Cucumber virus 3 . 4:0 I°5 I0°0 0°56 


(After Knight and Stanley) 
Tomato Bushy Stunt Virus 
This virus, first described by Smith (1935), was also the first virus 
to be isolated in a three-dimensional crystal form (Bawden and Pirie, 
