A MODEL-BASED APPROACH FOR DETERMINING ORIENTATIONS OF BIOLOGICAL MACROMOLECULES IMAGED BY CRYOELECTRON MICROSCOPY

A polar Fourier transform (PFT) method is de scribed that facilitates determination and refinement of orientations of individual biological macromolecules imaged with cryoelectron microscopy techniques, A three -dimensional density map serves as a high signal-to-noise model from w hich a PFT database of different views is generated and against which the PFTs of individual images are correlated, The PFT produces rotatio n-invariant data particularly well-suited for rapid and accurate deter mination of orientation parameters. The method relies on accurate know ledge of the center of symmetry and radial scale of both model and ima ge data but is insensitive to the relative contrast and background val ues of these data. Density maps may be derived from a variety of sourc es such as computer-generated models, X-ray crystallographic structure s, and three-dimensional reconstructions computed from images. The PFT technique has been particularly useful for the analysis of particles with icosahedral symmetry and could be adapted for the analysis of sin gle particles of any symmetry for which a crude model exists or can be produced. (C) 1996 Academic Press, Inc.