Low-resolution models for ribosomal particles reconstructed from electron micrographs of tilted two-dimensional sheets.

Models of the whole ribosome (70S) and its large subunit (50S) were obtained at low resolution (47 and 28 A respectively) by three-dimensional image reconstruction using diffraction data collected from electron micrographs of two-dimensional ordered arrays. The comparison of the various reconstructed images, using interactive computer graphics, enabled the assessment of the reliability of the method, the derivation of the shape of the small subunit (30S) and the assignment of several functional features such as the probable path taken by the nascent protein chain, the presumed site for the process of biosynthesis of proteins, and a feasible mode for tRNA binding. The reconstructed models of the various ribosomal particles may be used for phasing of X-ray diffraction data at low resolution.