ANIMAL VIRUSES 481 



with a diameter of 45 mju, (Williams, 1953) and a particle weiglit of '^47 X 10" 

 (Neurath et al., 1941). 



These units are obtainable from extracts of the keratinized masses of 

 papillomas (see Beard et al., 1955). About 1 mg. of virus material can be 

 isolated from 1 gm. of papilloma material through purification by alternate 

 high- and low-speed centrifugation. Earlier ultracentrifuge, diffusion, electron 

 microscope, and electrophoretic studies suggested that such virus prepara- 

 tions were comparatively pure (Neurath et al., 1941; Sharp et al., 1942a,b). 

 But later it was shown that they stiU contained an impurity of unknown 

 chemical composition (Schachman, 1951). Since the infectivity test available 

 is relatively insensitive, it is difficult to determine whether all of the charac- 

 teristic particles are infective. 



Chemical studies (Taylor et al., 1942; see Beard et al., 1955) on such material 

 showed that essentially only nucleic acid and protein were present. Treat- 

 ment with lipid solvents led to the extraction of some 1.5 % Upid material, 

 possibly an impurity. There was no evidence of carbohydrate in excess of 

 that expected from the nucleic acid. 



The nucleic acid content is about 8.7 % and, in contrast to poliovirus, is 

 exclusively of the deoxyribose type (DNA). From the DNA content and the 

 particle weight of 47 X 10", one can conclude that the entire nucleic acid of 

 the papilloma virus is represented by a single DNA molecule, assuming a 

 molecular weight of 4 X 10" (see Jordan, 1955). There has been no detailed 

 study of this DNA. Nothing is known concernmg the locahzation of the DNA 

 in the virus particle. 



Studies on the amino acid composition of the protein fraction have been 

 made on two preparations, using microbiological assays (Knight, 1950; see 

 Knight, 1954). Only the L-isomers of the amino acids were found. In contrast 

 to the protammes and histones, which are coupled to sperm DNA and 

 contain a large proportion of basic amino acids, a preponderance of the 

 acidic amino acids was detected in the protein of papilloma virus. 



Substances such as the S antigens, which could be distinguished from the 

 above-described units, have not been observed m papillomatosis. 



3, Equine Encephalomyelitis Virus 



The spherical infective particles of equine encephalomyelitis are about 

 40-50 m/Lt in diameter (Sharp et al., 1943; see Beard, 1948). A particle weight 

 of 152 X 10" has been calculated from physicochemical measurements 

 (Taylor et al., 1943) but this value is difficult to brmg into agreement with 

 the diameter, as determined by electron microscopy. A value of about 

 50 X 10" would seem to be a better approximation for a virus particle of the 

 observed size. 



