The interplay between X-ray crystallography, neutron diffraction, image reconstruction, organo-metallic chemistry and biochemistry in structural studies of ribosomes

Summary m Crystals of ribosomes, their complexes with components of protein biosynthesis, their natural, mutated and modified subunits, have been subjected to X-ray and neutron crystallographic analyses. Electron miscroscopy and 3-dimensional image reconstruction, supported by biochemistry, genetic, functional and organo-metailic studies were employed for facilitating phasing of the crystallographic data. For example, a monofunctional multi heavy-atom cluster (undecagold) was designed for covalent and quantitative binding to ribosomes. The modified particles were crystallized isomorphously with the native ones. Their difference-Patterson maps contain indications for the usefulness of these derivatives for subsequent phasing. Models of the ribosome and its large subunit were reconstructed from tilt series of 2-dimensional sheets. The comparison of the various reconstructed images enabled an initial assessment of the reliability of these models and led to tentative assignments of several functional features. These include the presumed sites for binding mRNA and for codon-anticodon interactions, the path taken by the nascent protein chain and the mode for tRNA binding to ribosomes. These assignments assisted in the design of biologically meaningful crystal systems. The reconstructed models are being used to identify structural features in initial density maps derived from X-ray and neutron diffraction data.