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Abstract:
It is known that in tobacco mosaic virus (TMV) structurally equivalent protein sub-units lie in
helical array about the long axis of the rod-shaped particle. It has not, however, proved possible
to make a reliable determination of the helical parameters by the usual methods of direct measurement
on X-ray fibre diagrams.
A quantitative comparison has now been made of the equatorial intensities in fibre diagrams of
TMV and of a mercury-substituted TMV, TMV-Hg. This has led to the determination, first of the
radial distance of the substituted mercury from the particle axis in TMV-Hg, and then of the parameters of the helical arrangement of protein sub-units in TMV. It also made possible the calculation of the radial density distribution in TMV, the result obtained being in good agreement
with the earlier work of Caspar.
From a knowledge of the helical parameters and other physical and chemical data it can be shown that the symmetry of TMV is that of the simplest possible helical line group, there being no symmetry elements other than the helical axis.
