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X-ray diffraction studies of the structure 

 of the protein of tobacco mosaic virus 



ROSALIND E. FRANKLIN 



Crystallographic Laboratory, Birkbeck College, 

 University of London 



A systematic application of the methods of X-ray diffraction to the study 

 of the structure of tobacco mosaic virus (TMV) has provided some informa- 

 tion concerning the general structural characteristics of the protein of the virus. 



Well -orientated specimens of concentrated TMV solutions can be obtained 

 in fine glass capillaries, and give excellent X-ray fibre-diagrams (Bernai and 

 Fankuchen, 1941; Franklin, 1955«). Fibre-diagrams which are similar but 

 significantly different may be obtained from heavy-atom derivatives of TMV 

 (Caspar, 1956; Franklin and Holmes, 1956), from the repolymerized nucleic 

 acid-free virus protein (Franklin, 19556) and from different strains of the 

 virus (Franklin, 1956a). Structure analysis is based on a detailed quantita- 

 tive comparison of the X-ray diagrams obtained from all these substances. 

 The analysis so far carried out is based on only a small part of the X-ray 

 diagram, and the work is therefore still at a relatively early stage. 



It has been shown that the virus protein is in the form of structurally 

 equivalent sub-units set in helical array about the particle axis (Watson, 

 1954; Franklin, 1955a) and (Franklin, Klug and Holmes, 1956) that these 

 structural sub-units may be identified with the chemical sub-units established 

 by amino acid and end group analysis (e.g. Harris and Knight, 1952, 1955; 

 Schramm and Braunitzer, 1953; Niu and Fraenkel-Conrat, 1955). 



The protein sub-units lie on a helix of pitch 23 Â, and there are 49 sub- 

 units to every 3 turns of the helix (Franklin and Holmes, 1956). A large 

 part of the results so far obtained is concerned with the shape of these sub- 

 units. If the TMV particle were a simple uniform-density rod, of circular 

 cross-section, the simplest possible shape of the protein sub-units would be 

 one resembling a slice of cake. We find, however, that the density of the 

 TMV particle is far from uniform, and that the shape of the protein sub-unit 

 is not simple. The rod-shaped particle has a hollow core, of radius about 

 20 Ä (Caspar, 1956; Franklin, Klug and Holmes, 1956) and the shape of 

 each sub-unit is such that it protrudes at both the inner and outer surfaces. 

 The helical array of protuberances on the outer surface has already been 

 described (Frankhn and Klug, 1956). That on the inner surface has been 



