480 W. SCHAFER 



obtained. The ratio of physical particles to plaque-forming units is about 

 30, making it probable that the spherical particles are identical with the 

 infective unit. 



In such highly purified preparations of infective particles only nucleic acid 

 and protein were found (Schwerdt and Schaffer, 1955; Schaffer and Schw^erdt, 

 1955b; Schwerdt, 1957a). The quantity of carbohydrate determined by the 

 anthrone test corresponded to the amount expected from the nucleic acid 

 content. The high, dry-weight density (1.56-1.62), as well as the fact that 

 the virus infectivity was not affected by organic solvents, suggested that 

 lipids are absent. 



The nucleic acid present is ribonucleic acid (RNA), most elegantly shown 

 by the chromatographic demonstration of the appropriate bases (Table I). 

 The purified virus possesses 22-30 % RNA, as determined by the quantita- 

 tive orcinol test. This amount corresponds to a single unit of RNA, if one 

 assumes that the biologically active RNA molecule has a weight of '^2x 10^, 

 as found for tobacco mosaic virus (Gierer, 1957). 



The amino acid composition of the proteinfractionhasnotyetbeenanalyzed. 



Incubation of the virus with RNAase during the purification procedure 

 does not affect the virus uifectivity, suggesting that the RNA is surrounded 

 by protein. 



In addition to the infective particle, a slower sedimenting, noninfectious 

 unit recently has been isolated (Schwerdt, 1957a; Schwerdt, 1957b; Mayer 

 et al., 1957; Mayer, 1957). Its virus specificity is revealed only by serological 

 tests and, therefore, it can be considered as an S antigen, according to our 

 definition. The antigenic behavior of at least a fraction of these miits is 

 different from that of the infective particles. 



This S antigen can be isolated in the last stage of the purification process 

 — the ultracentrifugation in the sucrose density gradient — from the layer 

 above the virus particles. Appropriate serological tests showed that prepara- 

 tions obtained by this method were free of host-ceU antigen. By electron 

 microscopy the S antigen appears as a round, flattened, low-contrast particle, 

 of somewhat larger diameter than the infective particle. In contrast to the 

 latter it contains little or no nucleic acid. It seems to be composed essentially 

 of protein. 



The composition of poliovirus brings to mind that of the spherical plant 

 viruses. The resemblance between the noninfectious, nucleic acid-free particle 

 of turnip yeUow mosaic virus (Markham, 1951) and the S antigen of polio- 

 virus is striking. 



2. Rabbit Papilloma Virus 



The spherical particles of rabbit papilloma virus are larger than the 

 infective particles of poHovirus, but are nevertheless comparatively uniform, 



