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The information in mRNA is interpreted by the pairing of consecutive triplets of nucleotides (codons) in the mRNA with the complementary anticodon sequences of the corresponding tRNAs. This pairing takes place on the ribosome, and accuracy of translation also depends on matching of a specific tRNA (with the appropriate anticodon) to the corresponding amino acid by a set of enzymes called aminoacyl-tRNA synthetase (aaRSs). aa-tRNAs generated by the aaRS enzymes are delivered to the ribosome by a protein factor called elongation factor-Tu (EF-Tu). During translation, the ribosome moves along the mRNA, and tRNAs interact with the mRNA at three distinct sites, designated the A (aminoacyl) site, the P (peptidyl) site, and the E (exit) site (Figure 2.23). Each aa-tRNA is brought into the A site first, where its anticodon is tested for complementarity to the mRNA codon present at that site. If the anticodon is complementary to the codon, the tRNA is retained; if the anticodon does not match the codon, the tRNA is rejected, and a new aa-tRNA is brought into the A site. A match between the anticodon and the codon in the A site triggers the tRNA in the A site to interact with the tRNA already present in the P site. The P site tRNA carries the growing polypeptide chain, and the next step is transfer of the growing peptide chain from the P site tRNA to the A site tRNA. The amino acid carried on the A site tRNA is attached to the carboxyl end of the polypeptide, and the polypeptide (now 1 amino acid longer) is now linked to the A site tRNA. The now unattached P site tRNA moves to the E site of the ribosome and the A site tRNA (which now carries the polypeptide) moves to the P site of the ribosome, which results in an empty A site. Each tRNA retains contact with the mRNA through anticodon-codon pairing so that the mRNA moves through the ribosome in concert with the tRNAs. This results in placement of the next codon of the mRNA in the empty A site, which is now available for entry of another aa-tRNA. Movement of the unattached P site tRNA into the E site results in ejection of the previous unattached tRNA from the E site, which allows the next tRNA to enter the cycle. The details of this process are described below.

Figure 2.22 Crystal structures of a tRNA and the ribosome. (A) Structure of a tRNA. The anticodon loop is at the bottom, and the 3′ acceptor end, where the amino acid or growing polypeptide is attached, is at the top. (From Yusupov MM, Yusupova GZ, Baucom A, et al, Science 292:883–896, 2001. Modified with permission from AAaS.) (B) The two subunits of the ribosome separated and rotated to show the channel between them through which the tRNAs move. The 30S subunit is on the left, and the 50S subunit is on the right. The tRNAs bound at the A, P, and E sites are indicated in yellow, green, and purple, respectively. (From Cate JH, Yusupov MM, Yusupova G, et al, Science 285:2095–2014, 1999. Modified with permission from AAAS.)