Biochemistry · Enzyme
RNA polymerases I, II and III !
RNA polymerases copy DNA into RNA; human cells have three in the nucleus, each dedicated to a type of RNA.
Updated 2026-11-01
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Key facts
- Pol I (nucleolus): the 45S precursor of 28S, 18S and 5.8S rRNA.
- Pol II (nucleoplasm): mRNA and most snRNAs and microRNAs. Most sensitive to α-amanitin.
- Pol III: tRNA, 5S rRNA, U6 snRNA and other small RNAs. Intermediate α-amanitin sensitivity; Pol I is resistant.
- They synthesise 5'→3' without a primer and without proofreading (more errors than DNA polymerase, but not inherited).
- Mitochondria have their own single-subunit RNA polymerase (POLRMT).
- Also called
- Pol I, Pol II, Pol III
- EC number
- 2.7.7.6
- Genes
- POLR1A, POLR2A, POLR3A (and the other subunits)
- Structure (PDB)
- 8WB0
Regulation
- Pol II depends on general factors (TFIID/TBP, TFIIB, TFIIH…), Mediator and activators on enhancers.
- Phosphorylation of the RPB1 CTD (YSPTSPS repeats): serine 5 at initiation, serine 2 during elongation.
- Pol I and Pol III output rises with growth signals (mTOR, MYC) to build ribosomes.
Inhibitors and toxins
- α-amanitin (Pol II > Pol III; Amanita mushrooms)
- Actinomycin D (intercalator; mostly stalls Pol I)
- Rifampicin (bacterial RNA polymerase only)
- Fidaxomicin (Clostridioides difficile RNA polymerase)
Clinical notes
- Amanita phalloides poisoning causes, after a quiet period, acute liver failure through Pol II inhibition.
- Mutations in Pol III subunits cause hypomyelinating leukodystrophies; in Pol I and Pol III subunits, Treacher Collins syndrome.
- Rifampicin is selective because bacterial RNA polymerase differs from the human ones.
Related conditions
Related
- Transcription
- RNA processing (cap, poly-A tail and splicing)
- α-Amanitin
- Transfer RNA (tRNA)
- Central dogma of molecular biology
Test yourself
Think of the answer, then tap the question to see it.
Which RNA polymerase makes tRNAs and 5S rRNA?
RNA polymerase III.
How do the three RNA polymerases rank by sensitivity to α-amanitin?
Pol II is the most sensitive, Pol III intermediate and Pol I resistant.
What does the 3D model 8WB0 show?
Mammalian Pol II (pig, almost identical to human) transcribing, with a modified guanine nucleotide (W0F) bound next to the catalytic site.
Educational information, not medical advice. For any symptom, see a health professional.
Sources: POLR2A: DNA-directed RNA polymerase II subunit RPB1 (human) (UniProt, 2026); 8WB0: De novo transcribing complex 17 (TC17), the early elongation complex with Pol II positioned 17nt downstream of TSS (RCSB Protein Data Bank, 2026); RNA Polymerase I Transcription (Reactome, 2026); RNA Polymerase II Transcription (Reactome, 2026); RNA Polymerase III Transcription (Reactome, 2026); Textbook of Biochemistry for Medical Students, 6th ed. (Jaypee Brothers Medical Publishers (Vasudevan DM, Sreekumari S, Vaidyanathan K), 2011).
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Representative model, for educational purposes only.