FIGURE 9-12 Key sites of interaction in a ribosome in the elongation phase of translation. (a) A computer model of the three-dimensional structure of the ribosome including MRNA, tRNAS, and the nascent polypeptide chain as it emerges from the large ribosomal subunit. (b) A schematic model of the ribosome during translation elongation. See text for details. [(a) J. Frank, Cryo-electron microscopy as an investigative tool the nibosome as an example, BioEssays 23, 2001, 725-732, Key sites of interaction in the ribosome (a) Computer model Polypeptide Figure 2. O Reproduced with permission of John Wiley & Sons, Inc. 50S positioned in the ribosome so that the codon of the MRNA can interact with the anticodon of the tRNA. The key sites of inter- action are illustrated in Figure 9-12. The binding site for MRNA is completely within the small subunit. There are three bind- ing sites for tRNA molecules. Each bound (RNA bridges the 30S and 50S subunits, positioned with its anticodon end in the former and its anminoacyl end (carrying the amino acid) in the latter. The A site (for aminoacyl) binds an incoming ami- noacyl-tRNA whose anticodon matches the codon in the A site of the 30S subunit. As we proceed in the 5' direction on the mRNA, the next codon interacts with the anticodon of the tRNA in the P site (for peptidyl) of the 30S subunit. The tRNA in the P site binds the growing peptide chain, part of which fits into a tunnel-like structure in the 50S subunit. The E site (for exit) contains a deacylated tRNA (it no longer car- ries an amino acid) that is ready to be released from the ribo- some. Whether codon-anticodon interactions also take place between the mRNA and the tRNA in the E site is not clear. Two additional regions in the ribosome are critical for pro- tein synthesis. The decoding center in the 30S subunit en- sures that only tRNAs carrying anticodons that match the codon (called cognate tRNAs) will be accepted into the A site. The peptidyltransferase center in the 50S subunit is the site where peptide-bond formation is catalyzed. Recently, many laboratories, especially those of Thomas Steitz, Venkatraman Ramakrishnan, and Ada Yonath, have used X-ray crystallogra- phy to “solve" the structure of the ribosome at the atomic level. For this accomplishment these three scientists received the 30S 5' MRNA 3' b) Schematic model - NH, Growing polypeptide chain Deacylated TRNA released from E site Peptidyl- transferase center Decoding center Movement of ribosome Nohel Prize in Chemistry in 2009 The results of their elegant studies clearly show