Control of Transcription by Transcription Factors

Five Classes of DNA-Binding Domains in Transcription Factors:

  • Transcription factors (TFs) regulate gene expression by recognising specific DNA sequences and recruiting or blocking RNA polymerase and cofactors.

    • Specificity arises from DNA-binding domains (DBDs) that recognise base-pair patterns exposed in the major groove of DNA via hydrogen bonding, electrostatics and van der Waals forces.

      • Binding often causes local DNA bending, enhancing recruitment of RNA polymerase or other TFs.

        • DBDs can be modular, allowing combinatorial control of gene networks.

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Superclass 1 - Basic Domains:

  • Basic leucine zipper (bZIP) and basic helix–loop–helix (bHLH) motifs.

    • The basic region (rich in Lys and Arg) binds DNA; the leucine zipper/HLH region mediates dimerisation.

      • Bind DNA as dimers, typically recognising palindromic sequences (e.g. 5'-TGACGTCA-3').

  • Example:

    • c-Jun/c-Fos (AP-1 complex): regulates stress response genes; phosphorylation alters dimerisation.

      • MyoD (bHLH): activates muscle-specific genes by recognising E-boxes (CANNTG).

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Superclass 2 - Zinc-Coordinating Domains:

  • Use zinc ions (Zn²⁺) to stabilise a compact, folded structure; zinc does not contact DNA directly.

    • C2H2 zinc finger (classical type): Cys₂-His₂ motif coordinates Zn²⁺; an α-helix fits into the major groove.

      • Each “finger” recognises ~3 bp; multiple fingers bind contiguous sequences (e.g. Zif268 with 3 fingers binds 9 bp).

      • Modular and tunable — exploited in zinc-finger nucleases and TALENs.

    • C4 zinc fingers (nuclear receptors):

      • Two zinc ions each coordinated by four cysteines.

      • Found in steroid and thyroid hormone receptors, retinoic acid, vitamin D, PPAR, etc.

        • Function as ligand-activated transcription factors: ligand binding triggers conformational changes allowing DNA binding and recruitment of co-activators (e.g. HATs) or co-repressors (e.g. HDACs).

    • Other classes: PHD, RING, LIM, CCHC.


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Superclass 3 - Helix-Turn-Helix (HTH) Domains:

  • Ancient, ubiquitous DNA-binding motif found in prokaryotic repressors and activators and eukaryotic homeodomain proteins.

    • Structure: two α-helices separated by a short turn; the C-terminal recognition helix inserts into the major groove.

      • Bind as symmetric dimers, one per half-site (typically 3.4 nm apart, or one helical turn).

  • Examples:

    • trp repressor (bacteria): represses tryptophan biosynthesis genes in the presence of tryptophan.

    • λ repressor (CI) and Cro proteins (phage λ): control lysogenic/lytic switch.

    • Homeodomain proteins (e.g. HOX genes): control body-plan patterning; extra N-terminal arm increases minor-groove contacts.


❗DNA recognition involves both direct readout (base-specific H-bonds) and indirect readout (DNA shape, groove width, hydration pattern).


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Superclass 4 - β-Scaffold Factors with Minor-Groove Contacts:

  • Bind the minor groove, which provides less sequence-specific information but allows recognition of DNA shape or flexibility.

    • Often architectural proteins that induce or stabilise bends, facilitating assembly of higher-order transcription complexes.

  • Examples:

    • IHF (integration host factor) and HU in bacteria: bend DNA to aid recombination, replication, and transcription.

    • HMG-box proteins (e.g. SOX-9): bend or unwind DNA; recruit chromatin remodellers; important in sex determination and chondrogenesis.

    • TATA-binding protein (TBP): binds the TATA box via the minor groove; bends DNA ~80°, aiding pre-initiation complex formation.


❗Minor-groove binders exemplify indirect readout and topological control — crucial in nucleosome-packed chromatin.


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Superclass 5 - Other/Composite Factors:

  • TFs with mixed or atypical binding motifs (e.g. fork-head, winged-helix, AT-hook, HTH-zinc hybrid).

  • Some use protein–protein tethering to DNA rather than direct base recognition (e.g. Mediator-associated factors).


❗Modularity — TFs often combine multiple DNA-binding and activation/repression domains to integrate complex regulatory inputs.

Control of Transcription in E.coli:

Bacteria regulate transcription primarily at the initiation stage.

  • Key strategies:

    • Alternative σ-factors determine promoter specificity.

    • Repressors block RNA polymerase binding.

    • Activators enhance polymerase binding or isomerisation to the open complex.

  • The “ground state” of most σ⁷⁰ promoters is ON unless repressed.

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Alternative Sigma Factors:

  • σ-factors are interchangeable subunits that confer promoter recognition.

    • Each recognises specific −35 and −10 consensus motifs.

      • σ⁷⁰ – housekeeping genes.

      • σ³² – heat-shock response.

      • σ⁵⁴ – nitrogen starvation (requires ATP-dependent activators).


❗σ⁵⁴-RNAP forms a closed complex incapable of initiation without activator-driven conformational change (energy from ATP hydrolysis).

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Repressible and Inducible Systems:

  • Repressible system – gene off when ligand present.

    • trp operon: Trp binds repressor → conformational change → binds operator (between −35 and −10) → blocks RNAP binding.

      • Negative feedback maintains tryptophan homeostasis.

  • Inducible system – gene on when ligand present.

    • lac operon: The Lac repressor binds to the operator to block RNA polymerase, but allolactose binds the repressor, causing it to release the DNA and allow transcription to occur.

      • Allows utilisation of lactose only when glucose is scarce.

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Positive Control by CAP (Catabolite Activator Protein):

  • CAP (or CRP) binds cAMP when glucose is low and binds upstream of lac promoter.

    • Bends DNA and contacts α-CTD of RNAP to increase binding affinity.

      • Cooperative binding increases the effective local concentration of RNAP at the promoter (the “tethering effect”).

❗lac operon integrates two signals (glucose and lactose), and so is an early example of logic-gate-like regulation.

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Other Examples of Activation:

  1. σ⁵⁴ + NtrC (nitrogen starvation):

    • NtrC binds enhancer-like sites tens of bp upstream.

      • Communicates with RNAP-σ⁵⁴ via DNA looping aided by IHF.

        • ATP hydrolysis by NtrC remodels RNAP into an open complex.

          • Distinct from CAP: activation via conformational change in RNAP rather than tethering.

  2. MerR (mercury resistance):

    • Promoter spacing altered (19 bp instead of 17 bp) misaligns −35/−10 regions.

      • Mercury binding to MerR twists DNA, realigning motifs → RNAP can initiate.

        • Example of DNA-mediated activation via distortion of promoter topology.

  3. λ repressor (CI):

    • Acts as dual activator and repressor.

      • Represses cro gene but activates its own cI gene by stabilising RNAP–DNA open complex.

        • DNA-binding regulated by proteolysis during SOS response (RecA-dependent cleavage).

Control of Transcription in Eukaryotes:

All somatic cells share the same genome but express distinct subsets of genes.

  • The key regulatory level is initiation of transcription by RNA polymerase II.

    • RNA Pol I → rRNA genes (constitutive);

    • RNA Pol III → tRNA + small RNAs;

    • RNA Pol II → protein-coding genes (highly regulated).

  • Regulation is layered:

    1. DNA accessibility (chromatin structure).

    2. Transcription factor binding.

    3. Co-activator/co-repressor recruitment.

    4. Post-transcriptional processing and export.

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Structure of Eukaryotic Transcription Factors:

  • Usually modular: consisting of separate DNA-binding and activation (or repression) domains joined by flexible linkers.

    • DNA-binding domains are often basic (positively charged) for DNA interaction; whilst activation domains are often acidic (interacting with positively charged surfaces of mediator or histones).

      • Modular design allows domain swapping in experiments (e.g. fusing a Gal4 activation domain to a heterologous DNA-binding domain still activates transcription).

❗Some TFs have intrinsically disordered regions (IDRs) enabling dynamic interactions and phase separation within transcriptional condensates (localised hubs where components necessary for gene transcription are brought together).

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Example: Regulation of the GAL1 Gene in Saccharomyces cerevisiae:

  • Function: GAL1 encodes galactokinase, an enzyme required for galactose metabolism.

    • Its transcription is induced by galactose and repressed by glucose, allowing yeast to preferentially utilise the most energetically favourable sugar.


Key Regulatory Proteins:

  • Gal4:

    • A transcriptional activator containing a non-classical C₆ zinc cluster DNA-binding domain. It binds to upstream activating sequences (UAS) of GAL genes.

  • Gal80:

    • Acts as a repressor by binding to the activation domain of Gal4, thereby preventing recruitment of the transcriptional machinery.

  • Gal3:

    • Serves as a sensor and signal transducer. In the presence of galactose and ATP, Gal3 binds Gal80, sequestering it in the cytoplasm and relieving repression of Gal4.


Mechanism of Regulation:

  1. Absence of Galactose:

    • Gal80 binds Gal4, masking its activation domain.

    • GAL1 remains transcriptionally silent.

  2. Presence of Galactose:

    • Gal3–galactose–ATP complex binds Gal80 and removes it from Gal4.

      • The freed Gal4 activation domain recruits Mediator and RNA polymerase II, initiating transcription.

  3. Glucose Repression:

    • High glucose triggers the Mig1–Tup1 co-repressor complex.

      • Tup1 recruits histone deacetylases (HDACs), promoting chromatin condensation and transcriptional silencing, even if galactose is present.


Experimental Evidence:

  • Fusion experiments demonstrate the modular nature of transcription factors:

    • The Gal4 DNA-binding domain, when fused to a heterologous activation domain (e.g. from viral protein VP16), still drives GAL1 expression.
          → This proves that DNA-binding and activation domains act             independently, confirming the modular architecture of eukaryotic                 transcription factors.

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Chromatin and Transcriptional Accessibility:

Chromatin condensation and histone modification are major regulatory layers.

  • Active chromatin (euchromatin) is characterised by acetylated histones and nucleosome-free promoter regions, promoting gene transcription.

    • Inactive chromatin (heterochromatin) is marked by deacetylated and/or methylated histones and a compact structure, which suppresses gene expression.

      • Transcription factors can partially penetrate condensed chromatin to initiate crucial chromatin remodeling (e.g., through complexes like SWI/SNF), making DNA accessible.

  • Example: Overexpression of a specific transcription factor can lead to the local decondensation of chromatin specifically around its binding sites, facilitating gene activation.

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Gene activation and chromatin decondensation are reciprocal processes, meaning they mutually influence each other.

  • The Mediator complex acts as a central hub, linking transcription factors (TFs) to RNA Polymerase II (Pol II) and integrating multiple signaling inputs.

    • Enhancer–promoter looping brings distal regulatory elements into close proximity, facilitated by structural proteins like cohesin and architectural proteins (e.g., CTCF).

      • Epigenetic regulation, specifically the methylation of CpG islands near promoters, silences transcription either by blocking TF binding or by recruiting repressive complexes.