Eukaryotic & Prokaryotic Transcription Mechanics
| Enzyme / Element | Primary Physiological Function & Location | Pharmacological / Tox Inhibitor Target |
|---|---|---|
| RNA Polymerase I (Eukaryotic) |
Located exclusively in the **nucleolus**. Synthesizes the major ribosomal RNA transcripts: **28S, 18S, and 5.8S rRNA** (comprising the structural core of ribosomes). | Insensitive to α-amanitin. Highly sensitive to low doses of Actinomycin D. |
| RNA Polymerase II (Eukaryotic) |
Located in the nucleoplasm. Synthesizes **mRNA** (protein-coding blueprints), **snRNA** (essential for spliceosome machinery), and **miRNA** (gene silencing regulators). Synthesizes in the **5′ → 3’** direction; reads template **3′ → 5’**. No proofreading exonuclease. | α-amanitin (from *Amanita phalloides* death cap mushrooms) potently binds and halts Pol II. Induces severe hepatotoxicity, liver failure, and acute tubular necrosis. |
| RNA Polymerase III (Eukaryotic) |
Located in the nucleoplasm. Synthesizes **tRNA** (amino acid adaptors for translation) and the small **5S rRNA** subunit. | Inhibited only by exceptionally high concentrations of α-amanitin. |
| Prokaryotic RNA Pol (Core vs. Holoenzyme) |
A single large multi-subunit complex ($\alpha_2\beta\beta’\omega$) synthesizes *all* bacterial RNA classes. The **core enzyme** performs elongation. Binding of the **Sigma ($\sigma$) factor** forms the **holoenzyme**, enabling sequence-specific promoter recognition and initiation. | Rifampin / Rifabutin (binds $\beta$-subunit, sterically blocking RNA elongation; treats *M. tuberculosis*). Fidaxomicin (inhibits $\sigma$-factor binding/initiation; treats *C. difficile*). |
| Promoter Regions (TATA / CAAT / Pribnow) |
Asymmetrical DNA sequences upstream of the transcription start site (+1) where RNA polymerase and transcription factors assemble. Mutated promoters completely eliminate or severely diminish transcription frequency. | Eukaryotes: TATA box (-25), CAAT box (-75). Prokaryotes: Pribnow box (-10), -35 sequence. |
High-Yield Core Realities:
- The Coding vs. Template Alignment: During transcription elongation, RNA Polymerase uses the **Template (Antisense) strand** to guide synthesis. The newly generated pre-mRNA transcript is an exact exact chemical duplicate of the **Coding (Sense) strand**, with the sole exception that all thymines (T) are swapped for uracils (U).
- Prokaryotic Coupling & Polycistronic Architecture: Prokaryotes lack a nuclear membrane barrier. Consequently, **transcription and translation occur concurrently** (translation starts at the 5′ end of the mRNA while the 3′ end is still being synthesized by RNA Pol). Furthermore, bacterial mRNA is often **polycistronic**, meaning a single continuous transcript contains multiple open reading frames encoding multiple distinct proteins (e.g., the *lac* operon). Eukaryotic mRNA is strictly monocistronic.
- Prokaryotic Termination Pathways: Termination occurs via two specific mechanisms:
- Rho-Independent (Intrinsic): The RNA transcript forms a stable, GC-rich **hairpin loop** followed immediately by a string of Uracil (U) residues. The weak structural stability of the A-U hybrid base-pairing causes the transcript to dissociate and drop out of the catalytic pocket.
- Rho-Dependent: The hexameric **Rho ($\rho$) protein** helicase binds to a cytosine-rich upstream tracking site on the nascent RNA, migrates 5′ → 3′ chasing the polymerase, and physically unwinds the RNA-DNA hybrid once RNA Pol stalls at a termination sequence.
- The Three Essential Eukaryotic Pre-mRNA Modifications: Eukaryotic transcripts are not functional until they undergo three co-transcriptional processing events inside the nucleus:
- 5′ Capping: Addition of a **7-methylguanosine cap** via a unique 5′-to-5′ triphosphate bridge. Essential for protecting transcripts against 5′ exonuclease degradation and serving as the primary recognition anchor for ribosomal binding during translation initiation.
- 3′ Polyadenylation: Cleavage of the transcript at the polyadenylation signal (AAUAAA), followed by the template-independent addition of ~200 Adenine residues by **Poly(A) Polymerase**. Confers nuclear export capability and controls cytoplasm half-life.
- Splicing: **Spliceosomes** (composed of snRNPs, pronounced “snurnps”) systematically recognize consensus sequences at the **5′ splice site (GU)** and **3′ splice site (AG)**. A branch point Adenine attacks the 5′ splice site, forming a looping **lariat structure** that is excised alongside the introns, precisely fusing the coding exons together.
- Clinical Spliceosome Pathologies:
- Anti-Smith (Anti-Sm) Antibodies: Targeted directly against the core protein component of snRNPs. Highly specific diagnostic marker for **Systemic Lupus Erythematosus (SLE)**. Anti-U1 snRNP antibodies are concurrently diagnostic for **Mixed Connective Tissue Disease (MCTD)**.
- β-Thalassemia Mutations: Often triggered by point mutations at intron-exon junctions, creating cryptic splice sites. This disrupts the reading frame during splicing, destroying functional β-globin production.