70S

70S refers to a ribosome with a sedimentation coefficient of 70S, typical of prokaryotes and of chloroplasts in plants and algae. It consists of two unequal subunits that assemble to form a functional particle during protein synthesis: a 50S large subunit and a 30S small subunit. The 50S subunit contains the 23S and 5S ribosomal RNAs and proteins; the 30S subunit contains the 16S rRNA and proteins.

Together, the subunits form the decoding center for messenger RNA and the peptidyl transferase center that catalyzes peptide bond formation.

In distribution, 70S ribosomes are the standard in bacteria and chloroplasts. In contrast, eukaryotic cytosolic ribosomes are 80S, and mitochondria in many organisms harbor ribosomes around 55S.

Antibiotics commonly target 70S ribosomes, including aminoglycosides (such as streptomycin) that bind the 30S, tetracyclines that block tRNA binding, and chloramphenicol and macrolides that inhibit the 50S subunit.

Clinical and research relevance: The structure and function of the 70S ribosome underpin the mechanism of action of many antibiotics and inform studies of antibiotic resistance and ribosome biogenesis.

History: The designation 70S arises from sedimentation velocity experiments carried out with ultracentrifugation in the mid-20th century, using Svedberg units to describe particle size.