Ending translation, and how antibiotics block it

The elongation cycle repeats, codon after codon, until a stop codon moves into the A site. No tRNA has an anticodon that pairs with a stop codon, so something else has to end the process.

Termination

The three stop codons, UAA, UAG, and UGA, are recognized by a protein called a release factor. Its three-dimensional shape mimics a tRNA, which lets it fit into the A site.

The freed polypeptide finishes folding into its three-dimensional shape. Bacteria have two release factors that recognize stop codons, and eukaryotes have one.

Check yourselfWhat would happen if a cell’s release factors stopped working?

Ribosomes would stall at stop codons. Finished polypeptides would stay attached to the tRNA in the P site instead of being released, and the ribosomes wouldn’t be freed to translate other mRNAs.

Bacteria and eukaryotes compared

Translation works the same way in all living things: the same genetic code, the same three stages, and the same A, P, and E sites. The differences lie in the parts.

Antibiotics that block bacterial translation

An antibiotic is a substance made by one microorganism that inhibits the growth of others, such as the bacteria that cause disease. Many antibiotics work by jamming bacterial translation. Because bacterial ribosomes and translation factors differ from the ones in eukaryotic cytosol, some of these drugs stop bacterial translation while leaving ours alone. That’s what makes them safe to use against infections in people, pets, and livestock.

Different antibiotics strike different steps:

• Some keep charged tRNAs from binding to the ribosome.
• Some block the peptidyl transfer reaction, so no peptide bonds form.
• Some stop translocation, freezing the ribosome on the mRNA.
• Some make the ribosome misread codons, so it builds abnormal proteins.
• Some cause the polypeptide to be released early, leaving it too short to work.

Whatever the step, the result is the same: the bacterium can’t make the proteins it needs, so it stops growing.

Check yourselfWhy can an antibiotic that blocks bacterial ribosomes clear an infection without stopping translation in the patient’s own cells?

Bacterial 70S ribosomes differ from the 80S ribosomes in human cytosol. The drug binds and blocks the bacterial kind but not the human kind, so only the bacteria lose the ability to make proteins.