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The structure of the disk of tobacco mosaic virus protein at low resolution has been determined by X-ray crystal analysis. Signs for the three principal projections were found by isomorphous replacement, using a mercury derivative. The heavy-atom positions were located by interpretation of difference Patterson maps on the basis of the non-crystallographic 17-fold rotational symmetry of the disk, and of the packing of the disks determined in the preceding paper (Finch et al., 1974). The electron density in the corresponding three projections was computed to a resolution of 6 A. From the projections in the [100] and [010] directions, which are at right-angles to the 17-fold rotation axis, a three-dimensional electron density map has been calculated making use of the non-crystallographic symmetry. The procedure is similar to the three-dimensional image reconstruction technique used in electron microscopy. The map indicates that the subunits in the two rings face the same way and, hence, that the disk is polar. There are differences between the subunits in the two rings at high radius, which are presumably a consequence of the pairing interaction responsible for stabilizing the two-layer polar structure. The map has been compared with a low-resolution map of the intact virus, and certain common features can be identified, notably the site of the nucleic acid. |
