Item

Abstract

X−ray diffraction photographs from dahlemense strain of tobacco mosaic virus show that it is very similar to the common strain, but additional meridional and near−meridional diffraction maxima appear on layer lines halfway between those given by the common strain. These are interpreted as arising from a regular perturbation of the arrangement of the outer part of the protein subunit. A theoretical discussion of the effects of a periodic perturbation on the diffraction pattern is given with application to the present case. The period and amplitude of the perturbation are established; the perturbation repeats after 46 A (two turns of the helix); its amplitude at the outside of the particle (85 A radius) is about 3 A. The length of the ordered domains of the perturbation is estimated to be about 600 A, about one−sixth of the length of the particle. The perturbation epparently results from an axial interaction between the outside ends of the subunits which requires 8 smaller separation for optimum bonding than the pitch determined by the bonding in the interior of the helix. The net result is that chemically identical subunits are packed in 98 symmetrically distinct but quasi−equivalent environments, and the deformations are coordinated in such a way that there is a minimum change in the side−to−side bonding. Similar but less well−developed effects have been observed in U2 strein. This suggests that the quasi−equivalent bonding found in the dahlemense helix may reflect a potential bonding pattern for all tobacco mosaic virus−like structures. Furthermore, quasi−equivalent bonding may be a frequent feature in ordered structures built of protein molecules