Transcriptional Machinery and RNA Polymerase Regulation
How Cells Decide Which Genes Are Transcribed
Transcription at advanced resolution
RNA polymerases interpret promoter architecture, transcription factor input, chromatin state, and signal-dependent cofactors. Regulation occurs at initiation, promoter escape, elongation, pausing, termination, and chromatin reassembly.
Key components
Modes of transcriptional control
Activation
- Recruits or stabilizes transcription machinery
- Can act through enhancers and coactivators
Repression
- Blocks assembly or function of machinery
- Can recruit chromatin modifiers
Promoter-proximal pausing
In many metazoan genes, RNA polymerase initiates transcription but pauses shortly after start site clearance, creating a regulatory window for rapid induction.
What is the strongest mechanistic role of an enhancer?
Enhancers act cis-acting to modulate transcription, often over long genomic distances via looping.
Correct answer: It increases transcription from a distance through protein-DNA interactions
Why is promoter-proximal pausing biologically useful?
Paused polymerase provides a preloaded state for fast gene induction.
Correct answer: It allows rapid and synchronized release into productive elongation after signaling
Readouts for transcription studies
| Assay | Primary output |
|---|---|
| RNA-seq | Steady-state transcript abundance |
| PRO-seq | Nascent transcription |
| ChIP-seq for Pol II | Polymerase occupancy |
| Reporter assay | Promoter or enhancer activity |
| Single-cell RNA-seq | Cell-to-cell expression heterogeneity |
Which outcome most directly indicates a defect in transcriptional elongation rather than initiation?
Accumulation at the promoter with reduced gene body occupancy suggests impaired elongation or pause release.
Correct answer: Polymerase present at the start site but reduced gene body signal
Name one mechanism by which repressors reduce transcription.
Repressors often act indirectly by altering chromatin or directly by blocking transcription factor function.
Correct answer: They can recruit chromatin-modifying complexes or inhibit assembly of the initiation complex