rRNAn

rRNAn is a notational convention used to refer to the nth copy of ribosomal RNA genes in a genome. In bacteria and archaea, rRNA genes are typically organized in operons (rrn operons) that transcribe 16S rRNA, 23S rRNA, and 5S rRNA together. Many species carry multiple rrn operons, often labeled rrnA, rrnB, rrnC, and the individual operons can be indexed as rRNAn where n denotes the operon number. In eukaryotes, rRNA genes are arranged in tandem repeats and transcribed by RNA polymerase I (with 5S rRNA transcribed separately by RNA polymerase III), producing precursor transcripts that are processed into mature rRNAs such as 18S, 5.8S, and 28S (in plants and animals) and 5S rRNA.

Ribosomal RNAs encoded by rRNAn form the core structural and functional components of ribosomes. In bacteria, mature rRNAs are 16S (small subunit), 23S, and 5S (large subunit). In archaea and eukaryotes, the corresponding rRNA sets differ in size and processing, but all serve to construct the small and large ribosomal subunits essential for protein synthesis. Transcription and processing involve co-transcriptional processing of polycistronic transcripts in prokaryotes and complex processing and chemical modification in eukaryotes.

Copy number and sequence variation among rRNAn operons can influence growth rates and genome organization, though concerted evolution tends to homogenize operons within a genome. In phylogenetics and metagenomics, specific rRNA genes (such as 16S or 18S) are widely used as universal markers for taxonomic identification and evolutionary studies, with rRNAn providing a framework for discussing individual operon copies.