THE PHYSICAL PROPERTIES OF INFECTIVE PARTICLES 313 



pH, for example, the junctions cannot be discerned. The two observations 

 suggest that the virus has a structural anisotropy that provides an easy 

 direction for breakage and that all broken ends are geometrically equivalent. 

 This conclusion is not out of accord with the X-ray finding that the virus 

 particle is formed as a helix, since its pitch is so small that a break along the 

 helical grooves could not be distinguished from a break truly perpendicular 

 to the particle axis. 



X-ray analysis has provided details concerning the surface structure of 

 TMV that so far have been unconfirmed by electron microscopy owing, 

 perhaps, to inadequacies of teclmique in the latter method. The major 

 structural feature of the rods has been shown to be (Watson, 1954; Franklin, 

 1956a,b; Franklin and Klug, 1956) the existence of a helical structure for the 

 protein portion of the virus, in which there are a nonintegral number of 

 crystallographic substructures in one turn of the helix: 49 substructures in 

 three turns. This result casts doubt upon the likelihood that the cross- 

 sectional shape of the rod is hexagonal. In addition, the X-ray results indicate 

 that the surface of the particle is deeply grooved, with the grooves following 

 the pitch of the helix (1 turn per 23 A). This finding implies that the electron 

 microscope should show a periodic structure along the virus surface; a 

 prediction yet to be visually confirmed. 



e. Internal Structure. The internal structure of TMV has been investigated 

 by electron microscopy, X-ray diffraction, and by sedimentation techniques. 

 Electron microscopy of partially disintegrated TMV particles has helped to 

 reveal the localization of the ribonucleic acid (RNA) within the virus and the 

 form taken by its protein substructures. The first intimations of the localiza- 

 tion of the RNA were obtained from the segmented appearance of occa- 

 sional frozen-dried rods, the segments being linearly arrayed and comiected 

 with a fine, axial strand that was thought to be RNA (Rice et al., 1953). 

 Later, it was fomid that electron micrographs of alkali-degraded TMV 

 showed fine fibers, occasionally comiected axially with undegraded bits of 

 virus, which were of the right order of size to be strands of RNA (Schramm 

 et al., 1955). It remained for Hart (1955) to make a positive identification of 

 these fibers in experiments in which the TMV was degraded in a controlled 

 way with detergent, and subsequently suspended in distilled water. Each 

 undegraded bit of TMV was found to have a coaxially localized fiber pro- 

 truding from it, usually from one end only, which fibers were dissolved away 

 following the application of RNAase. The observations were not exact 

 enough to distinguish between an axial and a coaxial localization of the 

 RNA fibers. Quite recently Hart (1958) has fomid that the appearance of the 

 RNA fibers depends ujDon whether or not the partially degraded material is 

 suspended in distilled water or in an ionic solvent subsequent to degradation. 

 When aminonium acetate was used the RNA was apparently uncoiled, 



