RNA polymerases are enzymes that synthesize RNA from a DNA template during the process of transcription. They are crucial for gene expression and differ significantly between eukaryotes and prokaryotes.
RNA Polymerases in Eukaryotes
| Polymerase Type | Main Products | Mnemonic / Function | Cellular Location |
|---|---|---|---|
| RNA Polymerase I | rRNA (28S, 18S, 5.8S) | RAMPANT – most abundant | Nucleolus |
| RNA Polymerase II | mRNA, miRNA, snRNA | MASSIVE – largest transcript | Nucleoplasm |
| RNA Polymerase III | tRNA, 5S rRNA | TINY – smallest transcripts | Nucleoplasm |
Key Functions and Mechanisms
- RNA Polymerase II:
- Opens DNA at the promoter site.
- Lacks proofreading (↑ mutation risk).
- RNA Polymerase I: Synthesizes most ribosomal RNA within the nucleolus.
- RNA Polymerase III: Synthesizes tRNA and 5S rRNA outside the nucleolus.
RNA Polymerase Inhibitors and Clinical Correlates
| Drug / Toxin | Target | Effect / Clinical Outcome |
|---|---|---|
| α-Amanitin (from Amanita phalloides, death cap mushroom) | Inhibits RNA Polymerase II | Causes severe hepatotoxicity, GI upset (dysentery) |
| Dactinomycin (Actinomycin D) | Inhibits RNA polymerases (both) | Used as chemotherapy (e.g., Wilms tumor, rhabdomyosarcoma) |
| Rifampin / Rifabutin | Inhibits DNA-dependent RNA polymerase (prokaryotes) | Used to treat tuberculosis |
RNA Polymerase in Prokaryotes
- One RNA polymerase synthesizes all three RNA types (rRNA, mRNA, tRNA).
- Regulated by sigma factors to recognize promoters.
- Target for rifamycin antibiotics.
Key Points to Remember
- RNA polymerase II → mRNA (major transcription enzyme)
- α-Amanitin inhibits RNA pol II → liver failure
- Dactinomycin inhibits RNA polymerase in both prokaryotes & eukaryotes
- Rifampin inhibits bacterial RNA polymerase → used in TB treatment
- No proofreading function → potential transcription errors
Learning Objective
By the end of this session, medical students should be able to:
Differentiate between eukaryotic and prokaryotic RNA polymerases, their specific products, and clinical inhibitors affecting transcription.
