ribosomal RNA (rRNA)
/ ry-buh-SOH-mul AR-en-AY /
The ribosome is the cell's protein-making machine — the workbench where mRNA instructions get turned into actual proteins. You might assume such a machine is built mainly of proteins. The surprise is that its bulk and its working core are made of RNA. Ribosomal RNA is that RNA: it is the structural and catalytic heart of every ribosome.
An rRNA is a long RNA molecule that folds into an intricate three-dimensional shape and then teams up with dozens of proteins to assemble a ribosome's two subunits. But rRNA is not mere scaffolding. The chemical step that actually links amino acids together — forming the peptide bond — is carried out by the rRNA itself, not by any protein in the ribosome. In other words, the ribosome is a ribozyme: an enzyme whose catalytic part is RNA. In eukaryotes, rRNA is made and the ribosome partly assembled in a dense region of the nucleus called the nucleolus.
rRNA matters far beyond bookkeeping. Because protein synthesis is essential to all cellular life, ribosomes and their rRNA exist in every cell on Earth, and the rRNA sequence changes only very slowly over evolutionary time. That makes rRNA a kind of universal molecular clock: comparing rRNA sequences is how Carl Woese discovered the archaea and redrew the tree of life into three domains. It is also a favorite antibiotic target — several common antibiotics work by jamming bacterial rRNA without harming ours.
The antibiotic erythromycin clogs the bacterial ribosome by sticking to its rRNA, halting the bacterium's protein production. Our ribosomes have differently shaped rRNA, so the drug leaves our protein factories running.
Erythromycin stops bacteria by binding their rRNA, sparing our differently shaped ribosomes.
rRNA is not just structural padding — it is the catalyst. The ribosome forms peptide bonds using its RNA, making it the most important known ribozyme.