ELECTRON SHADOW MICROGRAPHY OF THE TOBACCO MOSAIC VIRUS PROTEIN.

Two improvements are described in the use of shadow electron micrography for the observation of particles of macromolecular dimensions. One involves the substitution of gold for chromium as shadowing metal. The thinner gold film that can be employed gives a truer representation of the shape of particles so small that shape and size are significantly altered by the thickness of the shadowing metal deposited on them. The other consists in metal-shadowing small particles deposited on a very smooth surface such as that of polished glass and the handling of this metal film as a replica of the glass surface and the particles resting on it. This technique permits the photography of particles whose direct observation is disturbed by the fine structure that is brought out by shadowing a collodion substrate. Application of these methods to the electron micrography of the tobacco mosaic virus protein shows that its fibrils are rods about 125A both in height and breadth. Though the rods appear segmented, these segments have not been found to have a length that is constant or a small integral multiple of an underlying unit.