316 H. K. SCHACHMAN AND R. C. WILLIAMS 



at the bottom of the grooves that are formed by the polymerization of the 

 protein submiits. The X-ray results, however, give no direct clue as to the 

 geometry of the association of the RNA polyimcleotides with the protein 

 helix. 



The general structural features described above appear to be common to 

 all forms of TMV-like material: native virus, polymerized A-protein, poly- 

 merized X-protem (Takahashi and Ishii, 1952), reconstituted virus, and 

 different strains of native virus. The particles of pure protein, of course, have 

 only suspending fluid in the space otherwise occupied by E.NA. The recon- 

 stituted virus has an X-ray pattern identical with that of intact virus except 

 for a reduced degree of distinctness at large scattering angles. All strains of 

 native virus examined appear essentially identical except for differences in 

 the heights of the scattering peaks in the radial distribution plots (Franklin, 

 1956b; Holmes and Franklin, 1958). These differences are presumably due 

 to differences in the numbers, or in the packing, of the amino acid residues in 

 the protem subunits. 



2. Tomato Bushy Stunt Virus 



Among the viruses of spherical shape, tomato bushy stunt virus (BSV), 

 first isolated by Bawden and Pirie (1938), has been the most carefully exam- 

 ined for its physical chemical projoerties. Most of these studies have been 

 made on preparations of virus which have been purified from the extracted 

 juice of infected plants by a series of cycles of alternate high- and low-speed 

 centrifugation (Stanley, 1940). 



Various molecular weights have been reported for BSV. From sedimenta- 

 tion equilibrium experiments, McFarlane and Kekwick (1938) obtained the 

 value, 7.6 X 10^. The combination of the sedimentation coefficient, 132 S 

 (Lauffer and Stanley, 1940), and the diffusion coefficient, 1.15 X 10~' 

 cm.^/sec. (Neurath and Cooper, 1940), gave 10.6 X 10^ for the molecular 

 weight. The intrinsic viscosity has also been combined with the sedimenta- 

 tion coefficient to give 9.9 X 10^ (Markham, 1953). In all of these 

 determinations, measured values between 0.73 and 0.74 cc./gm. were used 

 for the partial specific volume. Because of the relatively large content of 

 ribonucleic acid (17 %) in the virus, such values of the partial specific 

 volume seem high. A recent reinvestigation (Cheng and Schachman, un- 

 published) gave 0.712 cc./gm., a value in excellent agreement with that 

 inferred from its composition of amino acids and RNA (see Section II, 

 A, 5, h, Hi). Reexamination of the diffusion behavior in an instrument 

 equipped with Rayleigh interference optics gave D^q, „, = 1.26 X 10"'^ 

 cm.^/sec. Thus the molecular weight from sedimentation and diffusion 

 becomes 8.9 X 10^ (Cheng and Schachman, mipublished). This compares 



