▲ FIGURE 4-27 Low-resolution model of E. coli 70S ribosome. (a) Top panels show cryoelectron microscopic images of E. coli 70S ribosomes and 50S and 30S subunits. Bottom panels show computer-derived averages of many dozens of images in the same orientation. (b) Model of a 70S ribosome based on the computer-derived images and on chemical cross-linking studies. Three tRNAs are superimposed on the A (pink), P (green), and E (yellow) sites. The nascent polypeptide chain is buried in a tunnel in the large ribosomal subunit that begins close to the acceptor stem of the tRNA in the P site. [See I. S. Gabashvili et al., 2000, Cell 100:537; courtesy of J. Frank.]

▲ FIGURE 4-28 Structure of T thermophilus 70S ribosome as determined by x-ray crystallography. (a) Model of the entire ribosome viewed from the side diagrammed in Figure 4-26 with large subunit on top and small subunit below. The tRNAs positioned at the A (blue), P (yellow), and E (green) sites are visible in the interface between the subunits with their anticodon loops pointing down into the small subunit. 16S rRNA is cyan; 23S rRNA, purple; 5S rRNA, pink; mRNA, red; small ribosomal proteins, dark gray; and large ribosomal proteins, light gray. Note that the ribosomal proteins are located primarily on the surface of the ribosome and the rRNAs on the inside. (b) View of the large subunit rotated 90° about the horizontal from the view in (a) showing the face that interacts with the small subunit. The tRNA anticodon loops point out of the page. In the intact logical conditions. However, multiple interactions between the large and small rRNAs and general domains of tRNAs (e.g., the D and T^CG loops) stabilize the tRNAs in the A and P sites, while other RNA-RNA interactions sense correct codon-anticodon base pairing, assuring that the genetic code is read properly.

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