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In addition to a termination codon, termination of translation requires release factors. These proteins recognize the termination codons in the ribosomal A site and promote the release of the polypeptide from the tRNA and the ribosome from the mRNA. In E. coli, there are two translation release factors, called RF1 and RF2, that recognize specific termination codons; RF1 responds to UAA and UAG, and RF2 responds to UAA and UGA. Another factor, called RF3, helps to release these factors from the ribosome after termination. Eukaryotes have only one release factor, which responds to all three termination codons. Some types of bacteria and mitochondria also have only one release factor, but those that do generally use UGA to encode an amino acid and not as a termination codon (Box 2.4).

Figure 2.30 outlines the process of translation termination. After translation stops at the termination codon, the A site is left unoccupied because there is no tRNA to pair with the termination codon. The release factors bind to the A site of the ribosome instead. They then cooperate with EF-G and ribosome release factor (RRF) to cleave the polypeptide chain from the tRNA and release it and the mRNA from the ribosome. An attractive model to explain how this could happen is suggested by the observation that the release factors mimic aa-tRNA (Box 2.4). If the release factor is occupying the A site, then the peptidyltransferase might try to transfer the polypeptide chain to the release factor rather than to the amino acid on a tRNA normally occupying the A site. When EF-G then tries to translocate the release factor with the polypeptide attached to the P site, it may trigger a series of reactions that release the polypeptide. The role of ribosome release factor in this process is uncertain, but it might be involved in releasing the mRNA after termination.