272 ROSALIND E. FRANKLIN [15 



made apparent by recent work in which a helical projection of the virus struc- 

 ture was calculated from X-ray data. This projection (which will be de- 

 scribed in detail elsewhere) shows that the internal and external protuberances 

 do not occur opposite to one another, thus suggesting that the general lie 

 of the protein sub-units is not strictly perpendicular to the particle axis, but 

 somewhat skew to it. 



Independent evidence from three different parts of the X-ray diagram 

 indicates that the shape of the protein sub-units is such that their helical 

 packing results in a set of holes in the virus particle at a radial distance of 

 55-60 Â from the particle axis. The mean radius of the particle is about 

 75 Â, and the maximum radius about 90 Â. The nucleic acid lies at a radius 

 of 40 Â (Franklin, 19566). The X-ray results therefore suggest that the 

 nucleic acid might well be more accessible to reagents via the external groove 

 and the 55-60 Â holes than via the hollow central core of the particle. 



The X-ray results, considered in conjunction with the ratio of nucleic 

 acid to protein, make it appear probable (though not yet certain) that the 

 nucleic acid is in the form of a single molecular chain following a rather flat 

 helical path, and fitting into the protein structure in a rather compact manner. 

 The shape of the protein sub-units must therefore be such as to allow this 

 RNA molecular chain to pass between adjacent sub-unit layers. 



Thus, although it is not yet possible to define the shape of the protein 

 sub-units, we know that it must be such as to favour their helical packing 

 and yet to leave holes between neighbouring sub-units at a distance of 

 55-60 Â from the particle axis, to leave a hollow core of radius about 20 Â 

 in the particle, to provide some kind of gap through which the RNA mole- 

 cule may pass at a radius of 40 Â and to give a protuberance at both the 

 inner and outer surfaces of the particle. 



Our X-ray diffraction results also indicate that the direction of the poly- 

 peptide chains within the sub-units tends to be perpendicular to the particle 

 axis, and in a tangential rather than a radial direction. 



It may be of interest to compare the information obtainable from X-ray 

 fibre-diagrams of TMV and from X-ray diagrams of single crystals of globular 

 proteins. Because of the strongly uniaxial character of the TMV particle 

 and also, in part, because of its great stability, a limited amount of informa- 

 tion concerning the inner structure of the particle can be obtained with 

 relatively little effort. By a much more laborious quantitative analysis of 

 X-ray fibre-diagrams of TMV and its heavy-atom derivatives, analogous to 

 that being carried out by Perutz, Kendrew and co-workers on globular pro- 

 teins, it should be possible to determine in some detail not only the shape 

 of the particle and of its sub-units, but also the direction of the back-bone 

 chains of both the protein and nucleic acid components. Some progress 

 has already been made in this direction. On the other hand, the three- 

 dimensional diffraction picture of TMV, which must be derived from fibre- 

 diagrams only, will never be as accurate as that obtained from single crystals 



