THE PHYSICAL PROPERTIES OF INFECTIVE PARTICLES 321 



Recent X-ray diffraction studies (Klug et al., 1957a,b) have led to certain 

 revisions in the interpretation of older crystallographic investigations. 

 These studies have also furnished evidence for the existence of protein sub- 

 units within the virus particles. As yet the isolation of protein subunits of 

 this virus has not been reported. The RNA has been isolated and character- 

 ized by the Archibald method (Cohen and Schachman, 1957) and the mole- 

 cular weight, 0.5-1 X 10^, was found to be 3 to 5 times larger than that 

 inferred from enzymatic determination of the end groups of the polynucleo- 

 tide chains (Markham and J. D. Smith, 1952). This value for the molecular 

 weight indicates that there are between 25 and 50 RNA molecules per virus 

 particle. However, this number likely will be revised downward, judging 

 from the behavior of other ribonucleic acids, as even milder methods are 

 discovered for the isolation of RNA from the virus. Recently Kaper and 

 Steere (1959) isolated turnip yellow mosaic virus RNA which was infectious, 

 but no physical properties have been reported thus far. 



4. Poliovirus 



Although several animal viruses have been purified sufficiently to allow 

 significant investigation of their physical properties, only the smaller ones 

 appear to be homogeneous with respect to size. [An. exception to this general- 

 ization would be the tipula iridescent virus (Williams and Smith, 1957) if 

 insect viruses were to be included in the category of "animal" viruses.] 

 Poliovirus, while not studied nearly as extensively as some of the plant 

 viruses, has been the most precisely characterized of the animal viruses 

 since its isolation and identification in 1954. 



All evidence indicates that the particles of poliovirus are miiform in size. 

 They appear so in the electron microscope, when they are examined as single, 

 air-dried or frozen-dried particles, and in two-dimensional packed arrays 

 (Schwerdt et al., 1954). The diameter of single, air-dried particles is about 

 310A, but when they are frozen-dried, or when their center-to-center dis- 

 tance is measured in arrays, the diameter is 270A. This difference of dia- 

 meter may properly be attributed to flattening upon drying of the single 

 particle. The external form of the frozen-dried particle appears to be that of 

 a sphere (Schwerdt et al., 1954), in contrast to some of the small plant viruses 

 (Kaesberg, 1956). 



Poliovirus has been crystallized in fairly large, optically isotropic crystals 

 which generally have a cube octahedron habit (Schaffer and Schwerdt, 1955; 

 Steere and Schaffer, 1958). By use of the frozen replica technique (Steere, 

 1957) it has been possible to examine the type of packing and the shape and 

 size of the virus particles when within the crystal (Fig. 12). The packing has 

 been found to be face-centered cubic, with the particles appearing spherical 

 and having a diameter of 270A (Steere and Schaffer, 1958). The possibility 



