rRNA

Ribosomal RNA (rRNA) is a non-coding RNA that forms the core structural and catalytic components of ribosomes, the cellular machines that translate messenger RNA into protein. rRNA molecules pair with ribosomal proteins to build the small and large subunits and contribute to the catalytic center that enables peptide bond formation and coordinates mRNA decoding and tRNA binding. Because of its central role in translation, rRNA is essential for protein synthesis in all living cells.

The rRNA makeup of ribosomes varies among life’s domains. Bacteria and archaea typically contain a small-subunit rRNA (16S in most bacteria and archaea) and large-subunit rRNAs (usually 23S and 5S). Eukaryotic ribosomes use a small-subunit rRNA (18S) and a large-subunit set of rRNAs (28S, 5.8S, and 5S). Organisms also differ in the arrangement of rRNA genes and in how they are transcribed.

Biogenesis and processing: In bacteria, rRNA genes are often organized in operons and transcribed as a polycistronic precursor. In eukaryotes, most rRNA genes are transcribed by RNA polymerase I in the nucleolus, with 5S rRNA transcribed by polymerase III. Precursor transcripts undergo extensive processing, endonucleolytic and exonucleolytic trimming, and site-specific chemical modifications guided by small nucleolar RNPs. Mature rRNAs assemble with ribosomal proteins to form functional subunits and, in eukaryotes, are exported to the cytoplasm.

Significance: rRNA sequences are highly conserved and widely used for phylogenetic and taxonomic analyses (e.g., 16S and 18S rRNAs). Many antibiotics target bacterial rRNA or its assembly, exploiting differences from host ribosomes. Mitochondria and chloroplasts retain their own rRNA genes, reflecting their bacterial origin and contributing to organellar ribosomes.