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M03.03.03 RNA polymerases
RNA polymerase plays a critical role in the synthesis of RNA from a DNA template during transcription. Different mechanisms and enzymes are involved in the process of prokaryotes and eukaryotes. The structure and function of RNA polymerases vary between these organisms.
Prokaryotic RNA Polymerase
- Core Structure: The core prokaryotic RNA polymerase has the subunit structure α2ββ′.
- Initiation of Transcription: Requires a protein factor called sigma (σ) to bind to the promoter. This sigma factor is released immediately after transcription begins.
- Termination: Sometimes requires rho (ρ) factor for termination.
- Inhibitors: Prokaryotic RNA polymerase can be inhibited by:
- Rifampin: Inhibits transcription by targeting RNA polymerase.
- Actinomycin D: Binds to DNA, preventing transcription.
Eukaryotic RNA Polymerase
Eukaryotes have three distinct RNA polymerases, each responsible for synthesizing different types of RNA.
- RNA Polymerase I: Located in the nucleolus; synthesizes 28S, 18S, and 5.8S rRNA.
- RNA Polymerase II: Located in the nucleoplasm, hnRNA/mRNA and some snRNA are synthesized.
- RNA Polymerase III: Located in the nucleoplasm; synthesizes tRNA, 5S rRNA, and some snRNA.
Transcription Factors and Termination in Eukaryotes
- Initiation: Requires transcription factors like TFIID, especially for RNA Polymerase II, to initiate transcription.
- Termination: The requirements for termination of transcription in eukaryotes are less understood.
- Inhibitors: Eukaryotic transcription can be inhibited by:
- Actinomycin D: Inhibits transcription by binding to DNA.
- α-Amanitin: A toxin from certain mushrooms, that specifically inhibits RNA Polymerase II.
Comparison of Prokaryotic and Eukaryotic RNA Polymerases
| Feature | Prokaryotic RNA Polymerase (α2ββ′) | Eukaryotic RNA Polymerase (I, II, III) |
|---|---|---|
| Number of Polymerases | Single RNA polymerase | RNAP 1, 2, and 3 |
| Initiation Factor | Requires sigma (σ) factor | Requires transcription factors (e.g., TFIID) |
| Termination Factor | Sometimes requires rho (ρ) factor | No rho factor required |
| Inhibition | Rifampin, Actinomycin D | α-Amanitin (inhibits RNAP II), Actinomycin D |
| Location of Polymerase I | N/A | Nucleolus (synthesizes rRNA, except 5S rRNA) |
| Location of Polymerase II | N/A | Nucleoplasm (synthesizes hnRNA/mRNA and some snRNA) |
| Location of Polymerase III | N/A | Nucleoplasm (synthesizes tRNA, 5S rRNA, some snRNA) |
Points to Remember
- Prokaryotic RNA polymerase requires sigma (σ) for initiation and may require rho (ρ) for termination.
- Eukaryotes have three distinct RNA polymerases, each producing different RNA types.
- Transcription in both prokaryotes and eukaryotes can be inhibited by Actinomycin D.
- RNA Polymerase II in eukaryotes is specifically inhibited by α-amanitin, a toxin from mushrooms.
- Transcription factors like TFIID are required in eukaryotes for RNA Polymerase II to initiate transcription.
Bibliography
- Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., & Walter, P. (2014). Molecular Biology of the Cell. Garland Science.
- Lodish, H., Berk, A., Zipursky, S. L., Matsudaira, P., Baltimore, D., & Darnell, J. (2000). Molecular Cell Biology. W. H. Freeman.