The helical arrangement of the protein sub-units
in tobacco mosaic virus

X-ray diffraction studies!.? have established that in the tobacco mosaic virus (TMV) particle the
virus protein is in the form of structural sub-units set in helical array about the particle axis, and
that there are (very nearly*) 31+ 1 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 » has, however, remained uncer-
tain. It has been estimated as 1o! or 122, 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 » 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 ob-
tain the earlier estimates. It will be described in detail in Acta Crystallographica. It is based ona
detailed quantitative comparison of the X-ray scattering by normal TMV and by a mercury-sub-
stituted 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,, to the extent
of one Hg atom to about 20,000 molecular weight of virus. Since there is only one cystein residue in
each chemical sub-unit of virus protein it is to be expected that all mercury atoms occupy equi-
valent 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 fibre-
diagrams 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 + 1 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.

Since there is only one mercury site on each chemical sub-unit, this indicates that there are
49 chemical sub-units on 3 turns of the helix. Using this result, it is found that a self-consistent
interpretation of a substantial part of the X-ray diagram of TMV is obtained if it be supposed that
406 PRELIMINARY NOTES VOL. 21 (1956)

there are 49 sfructural sub-units in 3 turns of the helix. ‘There is thus strong evidence that the sub-
units determined by chemical methods on the one hand, by X-ray methods on the other, are
identical. ‘

We must now consider the implications of this conclusion with regard to the relationship between
the molecular weights of the sub-unit and of the complete virus particle. Taking the particle length
to be 3000 A,® there are 49 X 3000/69 = 2130 sub-units in the complete particle. If the molecular
weight of a sub-unit is 17,0001.*, this gives a total molecular weight of 36.2- 108 for the protein or,
assuming a nucleic acid content of 6°%', a molecular weight of 38.4: 108 for the virus particle. If the
sub-unit weight is 8,000 the particle weight calculated in this way is 40.7: 108,

Our results therefore suggest that the molecular weight of TMV is about 40: 10%. ‘Phis is in
good agreement with the values obtained by SCHRAMM AND BERGOLD’ using sedimentation and
diffusion measurements, and by OsTER, Doty Aanp ZimM® and OsTerR®, using light-scattering
measurements. On the other hand WiLLiams, BACKUS AND STEERE!, using a direct method in-
volving weighing and electron microscope measurements, found the particle weight to be 50-108.

We do not attempt, here, to decide between these conflicting values. We wish to stress, how-
ever, that, as far as the chemical and X-ray determination of the particle weight of TMV is con-
cerned, the value, 40-108, arrived at above refers to the RNA together with the protein sub-units
which follow the helical arrangement described. On the basis of the X-ray data alone we cannot
exclude the possibility that the virus particle contains small amounts of protein organised ina
different way. The combination of chemical and X-ray data, however, probably provides the most
accurate method of determining the weight of that part of the virus which follows the regular
helical arrangement.

This work is supported by the Agricultural Research Council.

RosaLinp E, FRANKLIN
Birkbeck College Crystallography Laboratory, London (England) K. C. Hotmes

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Received June 15th, 1956