Structural studies of spherical viruses.

Tomato bushy stunt virus (TBSV) is probably the most suitable of the simple viruses for atomic resolution structure analysis. It is built from 180 copies of a coat protein subunit (Mw 41,000) and a single piece of RNA (MW 1.5 X 106) (1-3). There may also be a single copy of a large internal protein (MW 87,000) (a). The coat subunits form a T=3 icosahedral surface lattice. The symmetry of this lattice is represented by the polyhedron in Fig. 1. There are twofold, threefold, and fivefold axes of icosahedral symmetry as well as local ("quasi") twofold and threefold axes. The icosahedrai threefold axes are actually locally sixfold. The subunit tips in TBSV are clustered near strict and local twofold positions, so that the surface shows prominent dimeric morphological units (Fig. 2). Electron microscopy (Fig. 3) and low-resolution x-ray diffraction (Fig. a) have revealed an important aspect of the quasi-equivalent packing in TBSV: the