ELECTRON TOMOGRAPHY OF MACROMOLECULAR ASSEMBLIES

Electron tomography is a method for three-dimensional reconstruction of a specimen, based on its electron microscopical projections which are produced by tilting the specimen in the microscope about a fixed axis. After determining a common coordinate frame, a tomogram is computed by weighted backprojection. Higher resolution can be obtained by applying a contrast transfer function (CTF) correction to the projections. The tomographic data is further analyzed by extracting structural motifs from the raw tomograms, by selective alignment, multivariate statistical analysis, classification, and class-averaging, resulting in an increased signal-to-noise ratio and substantial data reduction. The volumetric data analysis facilitates the separation of a heterogeneous population of motifs into its constituents, and the visualization of the motifs in the raw tomograms by reassembly, replacing low signal-to-noise instances with higher signal-to-noise representations