50S

The 50S subunit is the large component of the prokaryotic 70S ribosome. Its name comes from its sedimentation coefficient in Svedberg units. The 50S subunit associates with the 30S subunit to form the functional 70S ribosome, which drives protein synthesis in bacteria and many archaea.

The subunit contains two ribosomal RNA molecules, the 23S rRNA and the 5S rRNA, along with numerous ribosomal proteins. The 23S rRNA houses the peptidyl transferase center, the catalytic site that forms peptide bonds, and it contributes to the formation of the polypeptide exit tunnel through which the growing protein chain exits the ribosome.

In translation, the 50S subunit provides the peptidyl transferase activity and participates in translocation during elongation. The aminoacyl-tRNA enters at the A site on the 30S subunit, while the P and E sites are formed at the interface with the 50S during elongation, with the 50S subunit helping to move tRNAs and mRNA through the ribosome.

The 50S subunit is a common target for antibiotics. Macrolides, lincosamides, and chloramphenicol bind to the 50S and inhibit peptide chain elongation, either by blocking the peptidyl transferase center or by obstructing the exit tunnel. Linezolid binds to a site on the 50S near the A-site. Resistance mechanisms include methylation of 23S rRNA by erm genes, mutations in ribosomal proteins, and active efflux.

Evolution and nomenclature: The 50S subunit is found in bacteria and archaea; eukaryotic cytosolic ribosomes have a distinct large subunit called 60S. Mitochondria and plastids generally retain ribosomal components related to the bacterial 50S, reflecting their endosymbiotic origins.