There are three codons, UAG, UAA, and UGA, that have no corresponding tRNA molecule. These codons are called stop codons. Instead of interacting with tRNAs, the A site occupied by one of these codons is filled by proteins known as chain release factors. In the presence of these factors the newly synthesized polypeptide chain is freed from the ribosome, and the mRNA, tRNA, and the 30S and 50S ribosomal subunits dissociate (Fig. 8.9). Release factor 1 causes polypeptide chain release from UAA and UAG, and release factor 2 terminates chains with UAA and UGA. A third protein, release factor 3, cooperates with the other two to stop protein synthesis. When the A site is occupied by a release factor, the enzyme peptidyl transferase is unable to add an amino acid to the growing polypeptide chain and instead catalyzes the hydrolysis of the bond joining the polypeptide chain to the tRNA. The carboxyl (COOH) end of the protein is therefore freed from the tRNA, and the protein is released.
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