RNA polymerase structure and function

RNA Polymerase - The Gene Scribe

• Function: Synthesizes RNA from a DNA template during transcription.
  • Forms a transcription bubble, unwinding DNA.
  • Synthesizes in the 5' → 3' direction; no primer needed.
• Prokaryotic RNA Polymerase: Single multisubunit complex.
  • Sigma (σ) factor: Recognizes promoter regions (e.g., Pribnow box).
• Eukaryotic RNA Polymerases:
  • RNA Pol I: Synthesizes rRNA (most numerous).
  • RNA Pol II: Synthesizes mRNA, snRNA, miRNA.
  • RNA Pol III: Synthesizes tRNA, 5S rRNA.

⭐ α-amanitin, found in Amanita phalloides (death cap mushrooms), is a potent inhibitor of RNA Polymerase II, causing severe hepatotoxicity.

📌 Mnemonic: Rampant, Massive, Tiny for Pol I, II, III products (rRNA, mRNA, tRNA).

Prokaryotic vs Eukaryotic Pols - Two Worlds, One Job

A single prokaryotic polymerase does it all, while eukaryotes delegate RNA synthesis to specialized enzymes.

The Transcription Cycle - A Scribe's Journey

• Initiation: RNA Polymerase holoenzyme (core enzyme + sigma factor) binds to the promoter region (e.g., TATA box at -10, -35 sequence) on DNA.
• Elongation: The polymerase unwinds DNA and synthesizes a complementary RNA strand, reading the template DNA 3'→5' and building the new RNA 5'→3'.
• Termination: Transcription halts at a terminator sequence. This can be Rho (ρ)-dependent or Rho-independent (hairpin loop formation).

⭐ Alpha-amanitin from Amanita phalloides (death cap mushrooms) potently inhibits RNA Polymerase II, causing massive liver failure. A key toxicology link.

📌 Rifampin Ruins RNA Polymerase (inhibits prokaryotic transcription).

Clinical Inhibitors - Poisoning the Scribe

• Rifampin:
  • Inhibits prokaryotic RNA polymerase by binding its β subunit.
  • Core drug in tuberculosis (TB) therapy.
  • 📌 Rifampin inhibits RNA polymerase.
• Actinomycin D (Dactinomycin):
  • Inhibits both prokaryotic & eukaryotic cells.
  • Intercalates in DNA, blocking RNA polymerase progression.
  • Used in cancer chemotherapy (e.g., Wilms tumor).
• α-Amanitin:
  • Toxin from Amanita phalloides (death cap) mushrooms.
  • Potent inhibitor of eukaryotic RNA polymerase II.

⭐ α-Amanitin specifically halts mRNA synthesis by inhibiting RNA Pol II, causing severe, often fatal, hepatotoxicity. It does not affect prokaryotic RNA polymerase.

High‑Yield Points - ⚡ Biggest Takeaways

• Prokaryotes have a single RNA polymerase; eukaryotes utilize three distinct types (I, II, and III).
• RNA Pol I synthesizes rRNA (the most abundant RNA), while RNA Pol III synthesizes tRNA.
• RNA Pol II synthesizes mRNA and is highly sensitive to inhibition by α-amanitin.
• The antibiotic Rifampin specifically targets the beta subunit of prokaryotic RNA polymerase.
• α-amanitin, a toxin from death cap mushrooms, causes severe liver toxicity by inhibiting RNA Pol II.
• In prokaryotes, the sigma (σ) factor is essential for promoter recognition and transcription initiation.