Learning Objectives
- Differentiate between the functions of RNA Polymerases I, II, and III.
- Understand the two-step transesterification process of pre-mRNA splicing.
- Identify the clinical significance of snRNPs and related autoimmune conditions (e.g., Lupus).
- Master the GT-AG rule for splice site recognition.
1. Eukaryotic RNA Polymerases
Eukaryotes utilize three distinct RNA polymerases, each responsible for synthesizing different types of RNA. They are numbered in the order that their products are used during protein synthesis.
| Type | Product | Location/Notes |
|---|---|---|
| RNA Pol I | rRNA (Ribosomal) | Nucleolus. Makes the 5.8S, 18S, and 28S subunits. |
| RNA Pol II | mRNA (Messenger) | Nucleoplasm. Also makes snRNA and miRNA. Inhibited by α-amanitin (Death Cap mushrooms). |
| RNA Pol III | tRNA (Transfer) | Nucleoplasm. Also makes 5S rRNA. |
2. The Splicing Mechanism
Splicing removes introns and joins exons. This is mediated by the Spliceosome, which consists of snRNPs (small nuclear ribonucleoproteins).
- The Process:
- The 5′ splice site (GU) is cleaved.
- The 5′ end of the intron loops back to the “branch point” (Adenine), forming a Lariat structure.
- The 3′ splice site (AG) is cleaved, and the two exons are ligated.
- The Rule: Introns almost always start with GU and end with AG.
Activity
3. Clinical High-Yield: snRNPs & Autoimmunity
Autoantibodies against spliceosome components are diagnostic markers for specific connective tissue diseases:
- Anti-Smith (Anti-Sm) Antibodies: Specific for Systemic Lupus Erythematosus (SLE). These antibodies target snRNPs.
- Anti-U1 RNP Antibodies: Highly associated with Mixed Connective Tissue Disease (MCTD).
4. Prokaryotic vs. Eukaryotic RNA Pol
- Prokaryotes: Use a single RNA polymerase (multisubunit complex) to make all three types of RNA.
- Rifampin: Inhibits prokaryotic RNA polymerase.
- Dactinomycin: Inhibits RNA polymerase in both prokaryotes and eukaryotes.
Activity