Symmetry of belladonna mottle virus: rotation function studies

Belladonna mottle virus, a spherical plant virus belonging to the tymovirus group, was crystallized by precipitation with polyethylene glycol 6000 in sodium citrate buffer (pH 5.6). The crystals belong to rhombohedral space group R3 (a \simeq 300 A, \alpha \simeq 60 °) with one molecule in the unit cell, and diffract X-rays to 3.5/~ resolution. Owing to the special interaxial angle of \simeq 60°, the lattice can also be described in terms of a pseudo-face-centred cubic cell. The face-centring vectors of the pseudo cell form the cell edges of the rhombohedral cell. The three-dimensional X-ray diffraction data on these crystals were collected using screenless oscillation photography to a resolution of 6 A. 37 842 independent reflections with {I/ \sigma (I) \geq 2-0} were measured on 51 filn ^pairs. The cell parameters were refined to a = 295.4A and \alpha = 59.86 ° by a postrefinement procedure. A rotation function was calculated using data between 11 and 13 A resolution. The function unambiguously reveals the particle icosahedral symmetry and orientation in the unit cell. The body diagonals of the pseudo-cubic cell are nearly tetrahedral. The icosahedral particle also has a set of tetrahedrally related threefold axes. The particle orientation is such that these two sets can be made coincident by a rotation of \simeq 180 ° about the rhombohedral [111] direction.