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5S ribosomal RNA is a small, roughly 120-nucleotide ribosomal RNA of the large subunit of cytoplasmic ribosomes. Its folded RNA contributes to the central protuberance, together with ribosomal proteins and parts of the larger ribosomal RNA. In bacterial ribosomes, it associates with the 50S subunit. The name refers to a sedimentation coefficient, not a count of five bases. Its conserved fold contains five helices and five loops.^[ref]

The role of a component can be tested by changing its physical independence. In a 2020 experiment, researchers joined a rearranged 5S sequence to the much larger 23S ribosomal RNA and removed the ordinary free 5S copies from Escherichia coli. Some engineered cells survived and their properly assembled ribosomes remained active in translation. However, defective large subunits accumulated. Cryo-electron microscopy showed malformed regions of the catalytic center in these misassembled particles.^[ref] Survival therefore distinguishes the need for the 5S sequence from the need to keep it as a separate RNA; it does not establish that assembly is unaffected.

In eukaryotes, 5S ribosomal RNA is transcribed by RNA polymerase III, unlike the major precursor transcribed by RNA polymerase I. The 1974 polymerase study identified the polymerase-III association through distinct inhibition responses of RNA synthesis in isolated nuclei and separated enzyme preparations.^[ref]

RNA polymerase I is a eukaryotic RNA polymerase specialized for synthesis of the large precursor from which the major ribosomal RNAs, including 18S, 5.8S and 28S in mammals, are processed. It acts in the nucleolus. The separate 5S ribosomal RNA is normally transcribed by RNA polymerase III, so not all ribosomal RNA has the same polymerase origin. ^[ref]

A yeast experiment disrupted the essential polymerase-I subunit RPA43. Driving ribosomal-RNA synthesis through a different polymerase could rescue the lethal defect, linking the essential requirement to RNA production. The rescue distinguishes the normal route from an experimentally engineered substitute, rather than implying that the alternative route is standard. ^[ref]

RNA polymerase III is a eukaryotic RNA polymerase that transcribes several small functional RNAs, including transfer RNA and 5S ribosomal RNA. It is distinct from RNA polymerase I, which makes the large precursor of most other ribosomal RNAs, and from RNA polymerase II, which transcribes protein-coding genes and many noncoding RNAs.

In 1974, Roberto Weinmann and Robert G. Roeder compared inhibition of RNA synthesis in isolated nuclei with the characteristic inhibitor sensitivity of separated RNA polymerases. The behaviour of transfer RNA and 5S ribosomal RNA synthesis matched RNA polymerase III, assigning particular RNA products to an enzymatic activity rather than grouping all nuclear transcription together.^[ref] This division of labour explains how production of translation machinery can be regulated separately from production of messenger RNA.

Robert G. Roeder investigates transcription and its regulation. His 1974 study with Roberto Weinmann helped identify which RNA polymerase produces transfer RNA and 5S ribosomal RNA. The researchers compared sensitivity to alpha-amanitin in transcription by isolated cell nuclei with the sensitivities of separated polymerase activities.^[ref]

Matching these responses supported the role of RNA polymerase III. The experiment connected production of particular RNA products to a biochemical activity, instead of assuming that every gene in a cell nucleus is transcribed by the same enzyme.

Roberto Weinmann coauthored a 1974 study assigning transcription of transfer RNA and 5S ribosomal RNA genes to RNA polymerase III. With Robert G. Roeder, he compared the effects of alpha-amanitin on RNA synthesis in isolated cell nuclei and on chromatographically separated polymerase activities.^[ref]

The logic was comparative: an inhibitor-response pattern for a defined RNA product could be matched to a separated enzyme activity. This linked a biochemical distinction between RNA polymerases to the kinds of genes they transcribe, rather than inferring function from enzyme abundance alone.