THE 70S ESCHERICHIA-COLI RIBOSOME AT 23 ANGSTROM RESOLUTION - FITTINGTHE RIBOSOMAL-RNA

Background: The ribosome - essential for protein synthesis in all orga nisms - has been an evasive target for structural studies. The best av ailable structures for the 70S Escherichia coli ribosome or its 30S an d 50S subunits are based on electron microscopical tilt experiments an d are limited in resolution to 28-55 Angstrom. The angular reconstitut ion approach, which exploits the random orientations of particles with in a vitreous ice matrix, can be used in conjunction with cryo-electro n microscopy to yield a higher-resolution structure. Results: Our 23 A ngstrom resolution map of the 70S ribosome elucidates many structural details, such as an extensive system of channels within the 50S subuni t and an intersubunit gap ideally shaped to accommodate two transfer R NA molecules. The resolution achieved is sufficient to allow the preli minary fitting of double-helical regions of an earlier three-dimension al ribosomal RNA model.Conclusions: Although we are still a long way f rom attaining an atomic-resolution structure of the ribosome, cryo-ele ctron microscopy, in combination with angular reconstitution, is likel y to yield three-dimensional maps with gradually increasing resolution . As exemplified by our current 23 A reconstruction, these maps will l ead to progressive refinement of models of the ribosomal RNA.