arnr

ARNr, or ribosomal RNA (rRNA), is a type of RNA that forms the structural and catalytic core of the ribosome, the cellular machinery for protein synthesis. All living cells contain rRNA.

In bacteria, the ribosome consists of a small subunit with 16S rRNA and a large subunit with 23S and 5S rRNA. In eukaryotes, the small subunit contains 18S rRNA, and the large subunit contains 28S, 5.8S, and 5S rRNAs. The genes for these rRNAs are typically encoded in tandem repeats and transcribed by RNA polymerase I (with the exception that 5S rRNA in eukaryotes is transcribed by RNA polymerase III). In most eukaryotes, the initial transcript is a single 45S pre-rRNA that is processed to yield 18S, 5.8S, and 28S rRNAs; 5S rRNA is transcribed separately and assembled with the others during ribosome biogenesis.

rRNA molecules contribute to the architecture of the ribosome and participate directly in the catalysis of peptide bond formation. The ribosome has binding sites for mRNA, tRNA, and various rRNA regions that form the catalytic center (peptidyl transferase activity) and the decoding center.

Ribosomal RNA genes are among the most conserved genetic elements and are used in phylogenetic analyses. For example, 16S rRNA sequences are standard markers for identifying bacteria and archaea, while 18S rRNA is used for eukaryotes. Variation within rRNA genes enables discrimination among species while remaining broadly comparable across life.

Clinically and practically, rRNA genes are targets for diagnostic sequencing (e.g., 16S rRNA gene sequencing to identify bacteria) and are affected by antibiotics that bind bacterial rRNA. Abnormal rRNA biogenesis or ribosome biogenesis defects can be involved in diseases and cellular stress responses.