Termination of Translation

Stop codons (UAA, UAG, UGA) in the mRNA signal termination of translation. When a stop codon enters the ribosomal A site, no corresponding tRNA exists to recognize it. Instead, release factors bind and catalyze hydrolysis of the bond between the polypeptide chain and the tRNA in the P site, releasing the completed protein.

Release factors are proteins that specifically recognize stop codons in the A site of the ribosome. They mimic the structure of tRNA but carry catalytic domains instead of amino acids. Upon stop codon recognition, release factors trigger peptidyl transferase activity to hydrolyze the ester bond linking the polypeptide to the P-site tRNA, facilitating protein release.

After the polypeptide is released, the ribosome must dissociate from the mRNA. Ribosomal subunits separate with the help of ribosome recycling factors and GTPase activity. The mRNA and deacylated tRNA are released, allowing the ribosomal subunits to be recycled for new rounds of protein synthesis.

Accurate termination ensures proteins are synthesized to their correct length and prevents read-through into untranslated regions. Improper termination can produce truncated or aberrant proteins that may misfold or aggregate. Proper termination also allows ribosomal subunits to be recycled efficiently, maintaining cellular protein synthesis capacity.

Prokaryotes use RF1 and RF2 to recognize different stop codons, plus RF3 for ribosome recycling. Eukaryotes use eRF1, which recognizes all three stop codons, and eRF3, a GTPase that enhances eRF1 function. Both systems ensure efficient and accurate termination of translation.

Premature stop codons can trigger nonsense-mediated mRNA decay, a quality control mechanism that degrades aberrant transcripts. The upf proteins and nonsense mediated mrna decay pathway detect stop codons in abnormal positions and mark mRNAs for degradation, preventing synthesis of truncated proteins that could harm cellular function.

GTP hydrolysis provides energy for release factor function and ribosome recycling. When release factors bind stop codons, GTP hydrolysis drives conformational changes that activate the catalytic domain to cleave the polypeptide-tRNA bond. Additional GTP hydrolysis by recycling factors powers ribosomal subunit dissociation and mRNA release.