56 AN INTRODUCTION TO THE STUDY OF VIRUSES 
cent of thymus nucleic acid in virus A and 3-7 per cent in virus B, and 
no ribose nucleic acid. But Knight (1947) found about 2:3 per cent 
of ribose nucleic acid present. 
Vaccinia Virus 
Compared with those plant viruses which have been studied and 
some animal viruses, vaccinia virus is very large and approaches more 
nearly the conception of an organism. 
Under the optical microscope, with the dark-ground field, the virus 
particles appear to be spheres, but electron micrographs (see Chapter 
VII) show the particles to be brick-shaped with a “nucleus” of denser 
material. They vary in size and may be about half as big as a small 
coccus. 
The chemical composition of vaccinia virus is 0-6 per cent P, 15-3 
per cent N, 2:2 per cent neutral fats, 2-2 per cent phospho-lipoid, 
and 1-4 per cent cholesterol, giving a total of 5-7 per cent lipoid. It 
is not certain that these last two are necessary constituents of the virus. 
There is present about 5-6 per cent nucleic acid; this nucleic acid is 
not acted upon by ribonuclease and gives the reactions for thymus 
nucleic acid. Adenine and guanine have been identified from hydro- 
lysates of the nucleic acid. There is a small quantity of copper present 
which does not seem to be an impurity since it cannot be removed 
by any mild method and is actually concentrated during the 
purification. The virus also seems to contain the enzymes 
phosphatase, catalase and lipase though it cannot be said for certain 
that these are part of the virus as the latter is capable of picking’ 
up these enzymes from solutions. Vaccinia virus is not acted on 
by trypsin. 
The rabbit papilloma virus of Shope is the smallest animal virus 
examined in any detail so far, and appears to be the simplest in 
constitution. 
The nitrogen content, 15 per cent, is indicative of a large proportion 
of protein, estimated as 90 per cent of the whole complex. It is not 
certain whether lipoid is an integral part of the virus, the largest 
amount extracted being 1-5 per cent. The nucleic acid is difficult to 
free from the virus and is relatively large in amount, about 8-7 per 
cent, and appears to consist wholly of the desoxypentose or thymus 
type (Beard, 1948). 
The two strains of equine encephalomyelitis virus, the eastern and 
the western, are said to contain nucleic acid only of the ribopentose 
