rRNAs

Ribosomal RNAs (rRNAs) are RNA components of ribosomes, the molecular machines that translate messenger RNA into proteins. Along with ribosomal proteins, rRNAs form the structural core of ribosomes and provide much of the catalytic activity needed for peptide bond formation and decoding. The major rRNA species differ among domains of life but share the essential role of enabling accurate and efficient protein synthesis.

In bacteria, the rRNA genes encode 16S, 23S, and 5S rRNAs. These genes are typically organized in operons and transcribed as a single precursor RNA, which is processed into mature rRNAs and assembled with proteins into the 70S ribosome (50S subunit containing 23S and 5S rRNAs, and 30S subunit containing 16S rRNA). In archaea, the rRNAs are 16S, 23S, and 5S as well, with transcription and processing that share features with both bacteria and eukaryotes.

In eukaryotes, the main rRNAs are 18S, 5.8S, and 28S in the large subunit, and 5S in the small subunit. Most rRNA genes are transcribed by RNA polymerase I as the 45S precursor, which is processed to yield 18S, 5.8S, and 28S rRNAs. The 5S rRNA is transcribed separately by RNA polymerase III and later combines with proteins to form the 60S large subunit and the 40S small subunit. The mature ribosome consists of a small 40S subunit and a large 60S subunit in the cytoplasm.

RRNA genes are highly conserved and widely used in phylogenetics and taxonomy. They are present in multiple copies per genome and are among the most abundant RNAs in cells.