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The surface of human adenovirus 2 was etched by irradiating intact virions with low-energy (1-keV) Ar+ ions in a Technics Hummer V sputter coater . Viral structures exposed by the etching process were shadowed and then examined in the electron microscope. Periods of etching that were sufficient to reduce the viral diameter by 20 to 30 nm revealed distinct substructural elements in the virion core. Cores were found to consist of a cluster of 12 large, uniformly size spheres which abutted one another in the intact virion. The spheres, for which we suggest the name " adenosomes ," had a diameter of 23.0 +/- 2.3 nm, and they were related to each other by two-, three-, and fivefold axes of rotational symmetry. The results support the view, originally suggested by Brown et al. (J. Virol. 16:366-387, 1975) that the adenovirus 2 core is composed of 12 large spheres packed tightly together in such a way that each is directed toward the vertex of an icosahedron . Such a structure, constructed of 23.0-nm-diameter spheres, would have an outside diameter (vertex-to-vertex distance) of 67.0 nm and a face-to-face distance of 58.2 nm. It could be accommodated inside the icosahedral adenovirus capsid if each large sphere were located beneath a capsid vertex.

Ion etching of bacteriophage T4 erodes virus components progressively from the outside to the inside while preserving the overall structure. The terminal portion of the T4 DNA molecule packaged can be specifically radiolabeled and was found to be eroded more rapidly than the remainder of the DNA. This strongly suggests that the first DNA to enter the prohead is condensed in the center of the capsid and is therefore shielded from the ion beam by the surrounding last packaged DNA. The results support a "spiral-fold" model for the arrangement of DNA within the icosahedral bacteriophage head. According to this model, phage T4 DNA strands run parallel to the long axis of the phage, with sharp (180 degrees) bends at the top and bottom of the capsid. The folds themselves are arranged radially about the long axis of the head in spirally organized shells.