When the mRNA stop codon is reached, the fully synthesized protein does not simply fall off the ribosome. Release factors (RFs) are the protein assistants that recognize the presence of a stop codon in the ribosomal A-site and trigger cleavage of the polypeptide from the P-site tRNA (Fig. 9). In prokaryotes, RF1 hydrolyzes the protein at stop codonsUAGandUAA, while RF2 recognizes stop codons UGA and UAA. Release factor RF3, a GTPase, stimulates the activities of the codon-specific RFs and also promotes the dissociation of RF1 or RF2 from the ribosome once the newly synthesized protein is released. In eukaryotes, only two RFs appear to trigger polypeptide release— one that combines the functions of RF1 and RF2 (recognizing all stop codons), and a GTPase like RF3.

The observation that prokaryotic RF1 and RF2 exhibit specificity for different RNA sequences suggests that direct contacts are made between the release factors and the mRNA. These protein–RNAinteractions contrast with the codon–anticodon contacts defined byWatson–Crick basepairing rules, and hint at a mechanism in which the proteins mimic bound tRNA.