Translation is the process within cells that converts genetic instructions encoded in messenger RNA (mRNA) into a functional chain of amino acids, known as a polypeptide. Protein synthesis must be precisely controlled to ensure the resulting protein has the correct length and sequence. Termination marks the final, regulated stage of this synthesis, signaling the ribosome to halt movement and release the completed polypeptide chain.
Essential Components for Termination
The signal for translation to stop is found directly within the mRNA sequence, consisting of three specific nucleotide triplets known as stop codons. These termination signals are universally recognized as UAA, UAG, and UGA. Unlike the sixty-one other codons in the genetic code, these three do not specify an amino acid and are not recognized by any transfer RNA (tRNA) molecule.
When a stop codon moves into the ribosome’s aminoacyl (A) site, it is recognized by specialized proteins called Release Factors (RFs). These RF proteins possess a structural similarity to tRNA molecules, allowing them to bind precisely into the A site and interact with the stop codon. This binding interrupts the standard elongation cycle and initiates the termination cascade.
The Three Stages of Termination
The termination process unfolds sequentially, beginning with the recognition of the stop codon by a release factor binding directly into the ribosomal A site. The large ribosomal subunit positions the release factor such that a motif, often containing Glycine-Glycine-Glutamine (GGQ), is near the growing polypeptide chain.
This positioning is the mechanism for the second stage, peptidyl-tRNA hydrolysis, which is the chemical cleavage that frees the protein. The release factor acts as a catalyst, promoting the addition of a water molecule to the bond linking the polypeptide chain to the final tRNA in the P site. This reaction takes place at the ribosome’s peptidyl transferase center (PTC), the site normally responsible for forming peptide bonds. The introduction of water instead of a new amino acid results in the scission of the bond.
The final stage is the release of the newly synthesized polypeptide into the surrounding cellular environment, such as the cytoplasm. Once the bond is hydrolyzed, the protein dissociates from the ribosomal complex. Following the protein’s exit, the release factor is ejected from the A site, often aided by a separate energy-dependent release factor. This leaves the ribosome in a post-termination state, ready for the final process of disassembly.
Differences Between Prokaryotic and Eukaryotic Systems
While the underlying mechanism of stop codon recognition and polypeptide hydrolysis is conserved across all life, the specific proteins that execute this function differ between prokaryotic and eukaryotic cells. Prokaryotes, such as bacteria, utilize two Class I release factors for stop codon recognition: Release Factor 1 (RF1) recognizes UAA and UAG codons, while Release Factor 2 (RF2) recognizes UAA and UGA.
A third protein, Release Factor 3 (RF3), functions as a GTPase, using the energy from Guanosine Triphosphate (GTP) to help remove RF1 or RF2 from the ribosome after polypeptide release. Eukaryotic cells employ a simpler system for stop codon recognition. They use a single factor, Eukaryotic Release Factor 1 (eRF1), which recognizes all three stop codons: UAA, UAG, and UGA.
Eukaryotic Release Factor 3 (eRF3) functions similarly to its prokaryotic counterpart, acting as a GTPase to stimulate the activity of eRF1 and facilitate its dissociation from the ribosome. The difference in the number of factors reflects an evolutionary divergence, but the functional result—the recognition of the termination signal and the hydrolysis of the peptide bond—remains the same.
Ribosome Recycling and Polypeptide Fate
Immediately following polypeptide release, the post-termination complex must be disassembled so its components can be reused for future rounds of synthesis. This process is known as ribosome recycling. In bacteria, the Ribosome Recycling Factor (RRF), along with the elongation factor EF-G, works to split the ribosome into its large and small subunits.
In eukaryotes, a different set of factors, including the protein ABCE1, facilitates the splitting of the 80S ribosome into its 40S and 60S subunits. This allows the ribosomal subunits to dissociate from the mRNA and the uncharged tRNA, making all components available to initiate translation again on a new message.
The newly released polypeptide chain is not yet a functional protein. The chain must fold into its correct three-dimensional structure, a process often guided by chaperone proteins that prevent misfolding or aggregation. Following folding, the protein may undergo further post-translational modifications, such as the addition of chemical groups or cleavage, before being transported to its final destination within the cell to begin its functional life.