Cryoelectron-microscopy image reconstruction of symmetry mismatches in bacteriophage phi 29

A method has been developed for three-dimensional image reconstruction of s ymmetry-mismatched components in tailed phages. Although the method describ ed here addresses the specific case where differing symmetry axes are coinc ident, the method is more generally applicable, for instance, to the recons truction of images of viral particles that deviate from icosahedral symmetr y. Particles are initially oriented according to their dominant symmetry, t hus reducing the search space for determining the orientation of the less d ominant, symmetry-mismatched component. This procedure produced an improved reconstruction of the sixfold-symmetric tail assembly that is attached to the fivefold-symmetric prolate head of phi 29, demonstrating that this meth od is capable of detecting and reconstructing an object that included a sym metry mismatch. A reconstruction of phi 29 prohead particles using the meth ods described here establishes that the pRNA molecule has fivefold symmetry when attached to the prohead, consistent with its proposed role as a compo nent of the stator in the phi 29 DNA packaging motor. (C) 2001 Academic Pre ss.