Figure 1. Cryoelectron microscopy of the Escherichia coli ribosome, with the small subunit (50S) in yellow, the large subunit in blue. The space between the subunits (intersubunit space) is where most of the ligands interact with the ribosome.

Figure 2. Morphology of Escherichia coli ribosomal subunits, and locations of ribosomal proteins, as determined by immunoelectron microscopy. Top row: Three-dimensional cryoelectron microscopy maps of (a) the 30S subunit, (b) the 50S subunit and (c) the 70S ribosome (Frank et al., 1995). Bottom row: Models of the two subunits and the 70S ribosome, as derived from visual interpretation of 2D projections from conventional electron microscopy of negatively stained specimen with approximate locations of some of the ribosomal proteins, marked by numbers. Locations of r-protein are determined by immunoelectron microscopy. Reproduced from Stöffler-Meilicke and Stöffler (1990) with permission by the American Society for Microbiology Press, Washington DC.

Figure 3. The elongation cycle, showing three-dimensional positions of tRNAs and elongation factors, as obtained by cryoelectron microscopy technique, overlaid on the 1.5-nm resolution map of the Escherichia coli 70S ribosome. Top: tRNA positions in the pretranslocational state. The aminoacyl-tRNA (pink) is present in the A site and peptidyl-tRNA (green), carrying a growing polypeptide chain, in the P site. Right: Posttranslocational state. After peptide bond formation, A- (pink) and P- (green) site tRNAs have moved to P (green) and E (yellow) sites, respectively, in an EF-G-dependent translocation reaction. EF-G (purple) momentarily interacts with ribosome to facilitate the translocation reaction. Bottom: Posttranslocational state with tRNAs occupying P and E sites. EF-G has been released, after GTP hydrolysis in GDP form, to vacate the overlapping binding site for the next ternary complex. Left: At this stage, a new aminoacyl-tRNA (grey, then pink) enters into the cycle in the form of a ternary complex with EF-Tu (red) and GTP, and binds to the ribosome in the A/T state. Recently, another E site, the E2 site (brown), has been suggested, in which the deacylated tRNA temporarily resides without maintaining codon–anticodon interaction. While the EF-Tu part of the ternary complex leaves the ribosome, following GTP hydrolysis, aminoacyl-tRNA moves to its proper A-site position to engage in peptide-bond formation, and deacylated tRNA is released from the E2 site.