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ARN polimerasas eucariotas
La ARN polimerasa (RNAP) se conserva en todos los animales, y las RNAP bacterianas, arqueales y eucarióticas comparten importantes similitudes de secu…
Eukaryotes have three different RNA Polymerases: RNA polymerase I, II, and III. They are structurally similar to one another and share common features with prokaryotic RNA polymerases, however, they transcribe different classes of RNA.
RNA Polymerase I transcribes most of the ribosomal RNA genes while RNA Polymerase III transcribes tRNA genes, some snRNA, and other small RNA genes.
The majority of protein-encoding RNA genes are transcribed by RNA polymerase II.
The carboxy-terminal domain of the RNA Polymerase II serves as a binding site for several transcription factors that regulate its enzymatic activity. The binding of these factors depends on the phosphorylation pattern of this domain.
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Q1: What are the three eukaryotic RNA polymerases and what do they transcribe?
Eukaryotes have three RNA polymerases with distinct roles. RNA Polymerase I transcribes most ribosomal RNA genes. RNA Polymerase III transcribes transfer RNA genes, some small nuclear RNA, and other small RNA genes. RNA Polymerase II transcribes the majority of protein-encoding genes. Though structurally similar to each other and to bacterial polymerases, each specializes in different RNA classes.
Q2: How does the carboxy-terminal domain of RNA Polymerase II regulate transcription?
The carboxy-terminal domain of RNA Polymerase II serves as a binding site for transcription factors that regulate its enzymatic activity. The binding of these factors depends on the phosphorylation pattern of this domain. This phosphorylation-dependent mechanism allows precise control of transcription initiation and elongation in eukaryotes.
Q3: What role do transcription factors play in eukaryotic RNA polymerase function?
All three eukaryotic RNA polymerases require specific transcription factors to function properly. These proteins remain attached to the RNA polymerase to guide the direction of RNA synthesis on the template DNA strand. The TATA-binding protein is common to all three polymerases, ensuring coordinated regulation of different gene classes.
Q4: How does poly(A) dependent termination work in eukaryotes?
Poly(A) dependent termination combines polyadenylation of the mRNA transcript with RNA Polymerase II termination. While the polymerase continues transcribing past the gene end, the transcript is cleaved at an internal site. The upstream part is released and receives a poly(A) tail, while a 5'-exonuclease digests the downstream product, helping the polymerase dissociate from the DNA template.
Q5: Why do eukaryotic RNA polymerases need to disassemble after transcription?
Once RNA strand elongation is complete, the RNA polymerase and associated transcription factors must disassemble and release the mRNA transcript. This release is essential for mRNA processing and export from the nucleus. Unlike bacterial polymerases that terminate at specific sequences, eukaryotic polymerases require coordinated cleavage and exonuclease activity to properly terminate and release the transcript.
Q6: How do eukaryotic and bacterial RNA polymerases compare structurally?
Eukaryotic RNA polymerases share significant structural and sequence similarities with bacterial RNA polymerase. RNA Polymerase II is most similar to bacterial RNAP in terms of structural organization and folding topologies of enzyme subunits. However, despite these structural similarities, their mechanisms of action differ substantially, particularly in transcription initiation, elongation, and termination processes.
Q7: What is the key difference between eukaryotic and bacterial transcription termination?
Bacterial genes contain specific DNA sequences that direct RNA polymerase to terminate at precise locations. In contrast, protein-encoding genes transcribed by eukaryotic RNA Polymerase II lack these specific termination sequences. Instead, eukaryotes rely on poly(A) dependent termination, which combines transcript cleavage with polyadenylation and exonuclease digestion to terminate transcription.