Fig. 9.

The T = 7d icosahedral surface lattice oriented as in the polyoma capsid cubic crystal lattice. The equivalent crystallographic twofold axes are along the x, y and z directions and the two classes of crystallographic threefold axes are marked by the open and filled triangles. The crystallographic asymmetric unit for the tetrahedral space group 123 consists of one pentavalent unit and five hexavalent units; in the icosahedral point group of the capsid the hexavalent units are, however, all equivalent. (The icosahedral asymmetric is ⅕ of the crystallographic one.) The five hexavalent units that occur in symmetrically different environments in the crystal lattice are marked α,β,γ,δ and ε. The β units in one capsid, adjacent to a crystallographic threefold axis, contact δ units of a neighboring capsid adjacent to the other class of crystallographic threefold axis. The α, γ and ε units, not involved in lattice contacts, are adjacent to and related by the single class of noncrystallographic threefold axis (open triangles located in the planes normal to the crystallographic twofold axes). This diagram provides a guide for the sections of the electron density map displayed in Fig. 10.