252 HEINZ SCHUSTER 



properties of TMV-RNA and, therefore, it is worthwhile to consider this 

 compound in some detail. 



1. Tobacco Mosaic Virus Ribonucleic Acid 



a. Arrangement of the Ribonucleic Acid in Tobacco Mosaic Virus 



TMV has also been studied in great detail with respect to structure, 

 both of the virus particle and of the RNA and protein components. The 

 virus particles are rods about 3000 A. 29 ' 30 long, with a packing diameter 

 of about 150 A. 31 Essentially uniform particles may be best obtained by 

 differential centrifugation. 32 ■ 33, 34 The RNA content per particle may be 

 determined by ultraviolet (UV) absorption after removal of the protein 35 

 or, more exactly, by determining the phosphorus content. A recent deter- 

 mination of the P content made by Knight and Woody 36 gave a value of 

 0.45% as an average of 19 samples, leading to a value of 5.1 % RNA. The 

 remainder of the particle is protein. 



The structure of the virus particle has been revealed in great detail by 

 X-ray crystallographic analysis. Figures 1 and 2 show a schematic represen- 

 tation of the structure of the virus protein and RNA. In order to obtain 

 the structure of the intact virus particle, one must conceive of the two fig- 

 ures superimposed so that the particle axes are the same. It was shown that 

 the virus protein is in the form of a helical array of structural equivalent 

 subunits, the pitch of the helix being 23 A. 37 The whole particle has a rather 

 deep helical groove following the line of the main protein helix 38 (packing 

 diameter = 150 A., maximum diameter ~180 A.). The radial electron 

 density distribution, which is approximately proportional to the mass den- 

 sity for biological substances consisting mainly of light atoms, has its strong- 

 est maximum at a radial distance of 40 A. for intact virus particles, 39 

 whereas RNA-free repolymerized A protein 40 has a pronounced minimum 

 at just that position. This strong density maximum at 40 A. in TMV must 



29 R. C. Williams and R. L. Steere, J. Am. Chem. Soc. 73, 2057 (1951). 

 3 " C. E. Hall, J. Am. Chem. Soc. 80, 2556 (1958). 



31 J. D. Bernal and I. Fankuchen, J. Gen. Physiol. 25, 111 (1941). 



32 H. K. Schachman, J. Am. Chem. Soc. 73, 4808 (1951). 



33 I. Watanabe and Y. Kawade, Bull. Chem. Soc. Japan 26, 294 (1953). 



34 H. Boedtker and N. S. Simmons, J. Am. Chem. Soc. 80, 2550 (1958). 



35 G. Schramm, H. Dannenberg, and H. Flammersfeld, Z. Naturforsch. 3b, 241 

 (1948). 



36 C. A. Knight and B. R. Woody, Arch. Biochem. Biophys. 78, 460 (1958). 



37 J. D. Watson, Biochim. el Biophys. Acta 13, 10 (1954). 



38 R. E. Franklin and A. King, Biochim. el Biophys. Ada 19, 403 (1956). 



39 D. L. D. Caspar, Nature 177, 928 (1956). 



40 G. Schramm, Z. Naturforsch. 2b, 112, 249 (1947). 



