High-resolution electron microscopy of the particles of tobacco mosaic virus

Abstract

High-resolution electron microscopy of the particles of tobacco mosaic virus has been undertaken in an attempt to elucidate the details of the surface contours of these objects. Some techniques have been developed to aid in the consistent attainment of resolutions better than 3 mμ in micrographs of shadowed virus particles. These developments bear upon the problems of mechanical stability of the specimen, and of contamination of the specimen in the electron microscope. The routine use of polystyrene latex particles is discussed in connection with the problem of the evaluation of the excellence of electron micrographs. Observations have been made of the surface structure of the particles of tobacco mosaic virus in normal preparations, and in freeze-dried, incinerated, and sonicated preparations. It is concluded that no evidence exists of periodic surface structure, either transverse or longitudinal. Sonicated preparations provide thin platelets which have been broken cleanly in planes transverse to the axis of the rods. When these platelets are observed perpendicular to their planes, they exhibit hexagonal cross-sections of varying degrees of perfection of figure. The failure of the normally observed rods of TMV to appear hexagonal is discussed. Evidence is given which indicates that some sort of creeping of the shadowing uranium film exists, and that consequently the surface appearance of rods lying on their sides on the substrate film has little bearing upon their true contours. It is suggested that a film of adsorbed oil from the vacuum pumps of the shadowing apparatus is the cause of the apparent surface mobility of the uranium atoms. It is concluded that the shape of the normal rods of tobacco mosaic virus is that of an elongated hexagonal prism.