Abstract
X-ray diffraction studies1, 2 have established that in the tobacco mosaic virus (TMV) particle thevirus protein is in the form of structural sub-units set in helical array about the particle axis, and that there are (very nearly a) 3n+ i such sub-units on three turns of the helix. The pitch of the helix is 23 A and the axial repeat period, therefore, 69 A. The value of n has, however, remained uncertain. It has been estimated as IO 1 or 123, giving 31 or 37 sub-units in the axial repeat period. We
now know that both of these estimates are false, and believe the correct value of n to be 16, giving 49 sub-units in 3 turns of the helix.
The method used to establish this result was altogether more rigorous than that used to obtain the earlier estimates. It will be described in detail in ,4 cta Crystallographica. It is based on a detailed quantitative comparison of the X-ray scattering by normal TMV and by a mercury-substituted
TMV, (Hg−TMV), kindly prepared for us by Dr. FRAENKEL−CONRAT. In this preparation
mercury was bound to the cystein residue of the virus protein, in the form −Hg−CH 3, to the extent of one Hg atom to about 2o,ooo molecular weight of virus. Since there is only one cystein residue in each chemical sub−unit of virus protein 4 it is to be expected that all mercury atoms occupy equivalent
sites in Hg−TMV, and hence that they all lie at the same radial distance from the particle
axis. Our X−ray diffraction measurements, on the equator and first five layer−lines of the fibrediagrams
of TMV and Hg−TMV, led us to conclude that all (or very nearly all) the mercury atoms
lie on a helix of radius 57 −I− i A, and, further, that in the axial repeat period of 69 A there are 3 turns of the helix and 49 equally spaced mercury sites
