Termination of Translation

Translation terminates when a stop codon—UAA, UAG, or UGA—enters the ribosome's A site. Unlike other codons, stop codons have no corresponding tRNAs. Instead, release factors RF1 or RF2 recognize the stop codon and trigger peptidyl transferase to add water rather than an amino acid, releasing the completed polypeptide chain into the cytoplasm.

Release factors RF1 and RF2 are proteins that bind directly to stop codons positioned in the ribosome's A site. This binding is specific to stop codons, which lack complementary tRNAs. Once bound, release factors catalyze the hydrolysis of the bond between the polypeptide and its tRNA, allowing the newly synthesized protein to be released.

RF3, a third release factor bound to GDP, joins the ribosome after RF1 or RF2 binding. On the ribosome, RF3 exchanges GDP for GTP, triggering a conformational change that causes RF1 and RF2 to dissociate. RF3 then hydrolyzes GTP, enabling the ribosomal subunits to separate from each other and from the mRNA.

The peptide exit tunnel is a water-filled channel within the large ribosomal subunit, approximately 10 nm × 1.5 nm in diameter. Its interior is not complementary to peptides, preventing the chain from sticking to tunnel walls. This allows the nascent polypeptide to slide through easily while passing through the tunnel toward the solvent side for folding.

Limited folding may occur within the peptide exit tunnel once the nascent chain reaches a spacious region, forming some α-helical structures. However, the majority of protein folding occurs after the polypeptide exits the ribosome and enters the cytoplasm. This post-translational folding is facilitated by molecular chaperones and protein folding machinery in the cellular environment.

After GTP hydrolysis by RF3, the ribosomal subunits dissociate from each other and from the mRNA. The disassembled subunits, bound to an initiator tRNA, are now available to join a new mRNA molecule and begin another round of translation, allowing the ribosome to be recycled for protein synthesis.

When a stop codon occupies the A site, release factors bind and redirect the peptidyl transferase enzyme to catalyze hydrolysis rather than peptide bond formation. This causes water to be added to the polypeptide-tRNA bond instead of a new amino acid. The resulting hydrolysis breaks the ester bond, releasing the completed polypeptide from its tRNA.