largesubunit

The large subunit (LSU) is the larger component of the ribosome, the molecular machine that translates messenger RNA into proteins. Working together with the small subunit, the LSU participates in peptide bond formation and the progression of translation. Across all domains of life, the catalytic activity of the LSU is primarily provided by ribosomal RNA, with proteins contributing to structure and interactions with other factors.

In bacteria, the LSU is the 50S subunit, composed of the 23S and 5S ribosomal RNAs and a complement of ribosomal proteins. In archaea, the large subunit is also around 50S with similar RNA components and proteins. In eukaryotes, the LSU is the 60S subunit, containing the 28S, 5.8S, and 5S rRNAs along with a larger set of ribosomal proteins. The 23S/28S-like rRNA within these subunits forms the peptidyl transferase center that catalyzes peptide bond formation, while the surrounding proteins contribute to structural support and factor interactions.

Functionally, the LSU houses the peptidyl transferase center and provides binding sites for transfer RNA at the A, P, and E positions. It collaborates with the small subunit to ensure correct codon–anticodon pairing, peptide elongation, and translocation during protein synthesis.

Biogenesis and variation are domain-dependent. In eukaryotes, assembly begins in the nucleolus before the subunits mature and export to the cytoplasm. Prokaryotic subunits assemble in the cytoplasm. Organellar ribosomes in mitochondria and chloroplasts often reflect their bacterial ancestry and can differ in size and composition from cytosolic ribosomes. Because the LSU is essential for translation, it is a common target for antibiotics that disrupt bacterial protein synthesis by binding to sites within the large subunit.