Procedures for detection and idealization of non-crystallographic symmetry with application to phase refinement of the satellite tobacco necrosis virus structure

An electron-density function with a region of local non-crystallographic symmetry is sampled with a movable symmetry grid. This provides a means of optimizing the parameters of the locally symmetric region as well as of idealizing its electron density through symmetry averaging. An iterative procedure for phase refinement uses local symmetry averaging and Fourier inversion, with selective reciprocal-space damping of unmodified function regions, and correction for solvent scattering. Application to the crystal-structure analysis of STNV, with 60-fold icosahedral non-crystallographic symmetry, shows good convergence and capability of phase extension from 10 to 4 Å resolution.