A MODEL OF PROTEIN-SYNTHESIS BASED ON CRYOELECTRON MICROSCOPY OF THE E-COLI RIBOSOME

THE ribosome is formed by assembly of proteins and nucleic acids, and synthesizes proteins according to genetic instructions in all organism s. Many of the biochemical steps of this fundamental process are known , but a detailed understanding requires a well-defined structural mode l of the ribosome. Electron microscopy combined with image reconstruct ion of two-dimensional crystals or single ribosomes(4) has been the mo st promising technique, but the resolution of the resulting models has been insufficient. Here we report a 25-Angstrom reconstruction of the ribosome from Escherichia coli, obtained by combining 4,300 projectio ns of ice-embedded single particles. Our new reconstruction reveals a channel in the small ribosomal subunit and a bifurcating tunnel in the large subunit which may constitute pathways for the incoming message and the nascent polypeptide chain, respectively. Based on these new fi ndings, a three-dimensional model of the basic framework of protein sy nthesis is presented.