The Escherichia coli large ribosomal subunit at 7.5 angstrom resolution

Background: In recent years, the three-dimensional structure of the ribosom e has been visualised in different functional states by single-particle cry o-electron microscopy (cryo-EM) at 13-25 Angstrom resolution, Even more rec ently, X-ray crystallography has achieved resolution levels better than 10 Angstrom for the ribosomal structures of thermophilic and halophilic organi sms. We present here the 7.5 Angstrom solution structure of the 50S large s ubunit of the Escherichia coli ribosome, as determined by cryo-EM and angul ar reconstitution. Results: The reconstruction reveals a host of new details including the lon g alpha helix connecting the N- and C-terminal domains of the L9 protein, w hich is found wrapped like a collar around the base of the L1 stalk. A seco nd L7/L12 dimer is now visible below the classical L7/L12 'stalk', thus rev ealing the position of the entire L8 complex. Extensive conformational chan ges occur in the 50S subunit upon 30S binding; for example, the L9 protein moves by some 50 Angstrom, Various rRNA stem-loops are found to be involved in subunit binding: helix h38, located in the A-site finger; h69, on the r im of the peptidyl transferase centre cleft; and h34, in the principal inte rface protrusion. Conclusions: Single-particle cryo-EM is rapidly evolving towards the resolu tion levels required for the direct atomic interpretation of the structure of the ribosome, Structural details such as the minor and major grooves in rRNA double helices and cc helices of the ribosomal proteins can already be visualised directly in cryo-EM reconstructions of ribosomes frozen in diff erent functional states.