Translation occurs in three distinct stages--initiation, elongation, and termination. Each step requires specific proteins that interact with the tRNAs, mRNA, and/or ribosomes. Some of these are listed in Table 27.4.

Termination of translation occurs when one of the stop codons (UAA, UAG, or UGA) appears in the A site of the ribosome. No tRNAs correspond to those sequences, so no tRNA is bound during termination.

Proteins called release factors (Table 27.4) participate in termination. RF1 binds to the ribosome when UAA or UAG is in the A site. RF2 binds when UAA or UGA is in the A site. RF3 is a GTPase that appears to stimulate the release process, via GTP binding and hydrolysis. The sequence of termination events is shown in Figure 27.26. It occurs as follows:

1. RF1 or RF2 binds to the ribosome near the A site. RF3 binds elsewhere.

2. The peptidyltransferase complex transfers the C-terminal residue of the polypeptide chain from the P-site tRNA to a water molecule, releasing the polypeptide chain from the ribosome.

3. The RF factors and GDP are released.

4. The tRNA is released.

5. The 70S ribosome is now unstable in the presence of a protein called ribosome recycling factors, as well as the initiation factors IF3 and IF1. Consequently, the 70S ribosome dissociates to 50S and 30S subunits and is ready for another round of translation.

When the ribosomal subunits separate, the 30S subunit may not always dissociate from its mRNA. For example, in polycistronic messages (more than one protein coded on an mRNA), the 30S subunit may simply slide along the mRNA until the next Shine-Dalgarno sequence (see here) and initiation codon are encountered and begin a new round of translation.