DNA-dependent RNA polymerase
Summary
DNA-dependent RNA polymerase is the enzyme responsible for synthesizing RNA from a DNA template, catalyzing transcription in both prokaryotes and eukaryotes. In eukaryotes, three distinct polymerases (I, II, III) transcribe different classes of RNA, while prokaryotes use a single multi-subunit enzyme. This enzyme is a key target for antibiotics (e.g., rifampin) and is essential for gene expression.
Detail
DNA-dependent RNA polymerase catalyzes the synthesis of RNA using a DNA template in the 5'→3' direction, reading the template strand 3'→5', without requiring a primer (unlike DNA polymerase). It unwinds DNA locally to create a transcription bubble and adds ribonucleotides complementary to the DNA template, incorporating uracil instead of thymine.
In prokaryotes, a single core RNA polymerase (subunits: 2α, β, β', ω) associates with a sigma factor to recognize promoter sequences (e.g., -10 and -35 regions) and initiate transcription. Rifampin and rifamycins inhibit bacterial RNA polymerase by binding the β subunit, making this a clinically important antibiotic target (used in TB treatment).
Eukaryotes have three nuclear RNA polymerases with distinct roles: - RNA Pol I: synthesizes most rRNA (28S, 18S, 5.8S) in the nucleolus - RNA Pol II: synthesizes mRNA and snRNA; requires a promoter (TATA box), uses general transcription factors (TFIID binds TATA box), and the product is capped (5' methylguanosine cap) and polyadenylated (3' poly-A tail); sensitive to α-amanitin (found in Amanita phalloides mushrooms), causing severe hepatotoxicity - RNA Pol III: synthesizes tRNA, 5S rRNA, and other small RNAs; most sensitive to high-dose α-amanitin
Mnemonic for α-amanitin sensitivity: "3 is for mRNA, 1 is for rRNA, 2 is for tRNA" — actually remembered as Pol I makes rRNA (I before mRNA alphabetically... not sensitive to amanitin), Pol II makes mRNA (very sensitive—mushroom poisoning target), Pol III makes tRNA (moderately sensitive).
Clinical relevance: Rifampin's mechanism is tested heavily for TB/leprosy treatment regimens; α-amanitin toxicity is a classic toxicology/pharmacology question involving mushroom poisoning leading to fulminant hepatic failure due to RNA Pol II inhibition and halted protein synthesis. Mitochondria have their own distinct RNA polymerase (not one of the three nuclear ones), important for understanding mitochondrial gene expression and inheritance patterns.
Sources
- First Aid for the USMLE Step 1
- Lippincott's Illustrated Reviews: Biochemistry
- Molecular Biology of the Cell (Alberts et al.)
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