Bacterial ribosome (50S subunit)
Also known as: 50S ribosomal subunit, bacterial 70S ribosome, 23S rRNA
The protein-making machinery of bacteria, built differently enough from the human ribosome to be a selective drug target. Macrolides such as azithromycin bind the 50S subunit and stop the bacterium making proteins, halting its growth.
- Kind
- other
- Also called
- 50S ribosomal subunit, bacterial 70S ribosome, 23S rRNA
- Where it is found
- The cytoplasm of bacteria; not present in human cytoplasm (mitochondrial ribosomes are related but distinct).
What it does
Every cell makes protein on ribosomes, but bacterial ribosomes (70S, made of 30S and 50S subunits) differ from human cytoplasmic ribosomes (80S) in their RNA and proteins, so antibiotics can bind them selectively. Macrolides (azithromycin, clarithromycin, erythromycin) bind the 23S ribosomal RNA in the exit tunnel of the 50S subunit and block the growing peptide chain from leaving, which stops protein synthesis; the effect is mainly bacteriostatic, leaving the immune system to clear the organism. Because macrolides reach very high concentrations inside cells, they are effective against intracellular organisms such as Chlamydia, Mycoplasma and Legionella that penicillins cannot reach. Other classes target the ribosome at different sites: tetracyclines and aminoglycosides on the 30S subunit, clindamycin and linezolid on the 50S. Human mitochondria have ribosomes of bacterial ancestry, which may contribute to some antibiotic toxicities. Resistance follows methylation of the binding site or efflux pumps.
Role in the body
Bacterial protein synthesis (translation); inhibition halts bacterial growth.