⚡ Core Concept: Transcription & Metabolic Transcriptional Flux
Transcription represents the initial enzymatic step in the expression of genetic information, catalyzing the synthesis of RNA molecules from a complementary DNA template. In cellular biochemistry, transcriptional rates are tightly coupled to nutrient availability, energy charge (ATP/ADP ratio), and metabolic intermediates through promoter architecture, sigma factor switching, and transcription factor phosphorylation cascades.
1. Prokaryotic vs. Eukaryotic RNA Polymerases
Prokaryotes utilize a single multi-subunit RNA polymerase core enzyme (α2ββ’ω, ~400 kDa) which associates with an interchangeable σ (sigma) factor to constitute the holoenzyme capable of sequence-specific promoter recognition at the -10 (Pribnow box: TATAAT) and -35 (TTGACA) consensus elements.
In contrast, eukaryotes deploy three distinct nuclear RNA polymerases with dedicated transcriptional tasks:
- RNA Polymerase I: Localized to the nucleolus; transcribes the 45S pre-rRNA precursor processing into 28S, 18S, and 5.8S rRNAs. Insensitive to α-amanitin.
- RNA Polymerase II: Localized in the nucleoplasm; transcribes all protein-coding precursor mRNAs, snRNAs (U1, U2, U4, U5), and microRNAs. Highly sensitive to α-amanitin (inhibited at 10-8 M). Features a unique Carboxy-Terminal Domain (CTD) composed of tandem repeats of the heptapeptide consensus
Tyr-Ser-Pro-Thr-Ser-Pro-Ser(YSPTSPS), whose dynamic phosphorylation coordinates 5′-capping, splicing, and 3′-polyadenylation. - RNA Polymerase III: Transcribes small non-coding RNAs including tRNAs, 5S rRNA, and U6 snRNA. Moderately sensitive to α-amanitin (inhibited at 10-6 M).
Figure 1: Catalytic Cycle of Eukaryotic RNA Polymerase II coupled with C-Terminal Domain heptapeptide phosphorylation.
2. Post-Transcriptional Processing of Pre-mRNA
Nascent pre-mRNA undergoes three obligatory biochemical modifications prior to export through the nuclear pore complex into the cytosol:
- 5′ 7-Methylguanosine Capping: Occurs co-transcriptionally when nascent RNA reaches 20–30 nucleotides. Triphosphatase removes the 5′-γ-phosphate; guanylyltransferase transfers GMP from GTP forming an unconventional 5′-to-5′ triphosphate linkage; guanine-7-methyltransferase adds a methyl group from S-adenosylmethionine (SAM) yielding m7GpppN cap, protecting transcripts from 5′ exonucleolytic degradation.
- Spliceosomal Intron Removal: Small nuclear ribonucleoproteins (snRNPs U1, U2, U4/U6, U5) assemble stepwise at the 5′ splice site (GU), branch point adenosine, and 3′ splice site (AG). Two consecutive transesterification reactions excise the intron as a branched lariat structure and ligate adjacent exons.
- 3′ Polyadenylation: Cleavage and Polyadenylation Specificity Factor (CPSF) binds the AAUAAA polyadenylation signal while Cleavage Stimulation Factor (CstF) binds a downstream GU-rich tract. Cleavage generates a free 3′-OH where Poly(A) Polymerase (PAP) synthesizes a homopolymeric tail of 200–250 adenine residues in a template-independent, ATP-dependent manner.
3. High-Yield Comparative Summary
| Parameter | Prokaryotic Transcription | Eukaryotic Transcription |
|---|---|---|
| Polymerase Machinery | Single RNA Pol (α2ββ’ω + σ) | Three distinct Pols (Pol I, II, III) |
| Promoter Motifs | -10 (Pribnow) & -35 consensus | TATA box (-25 to -30), Inr, BRE, DPE |
| Coupling with Translation | Simultaneous (polysomes form on nascent mRNA) | Uncoupled (transcription in nucleus, translation in cytosol) |
| Transcriptional Inhibitors | Rifampicin (binds β-subunit), Actinomycin D | α-Amanitin (Pol II specific), Actinomycin D |
