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Question 1

lec 5

Answer 1

Repressors → Repressors are the functional converse of activators, meaning they bind to DNA and inhibit transcription.

Example: Bacteriophage 434 repressor, which uses a helix-turn-helix motif to bind DNA in the major groove and block transcription.
DNA-Binding Domains → Different transcription factors (TFs) have specific DNA-binding motifs that help them bind to specific sequences in DNA.

Example Motifs:
C2H2 Zinc Finger → Most common in humans.
C4 Zinc Finger → Found in nuclear hormone receptors.
Leucine Zipper (bZIP) → Forms dimers and binds to DNA via hydrophobic leucines.
Basic Helix-Loop-Helix (bHLH) → Similar to bZIP but with a loop separating helices.
Dimerization → Transcription factors can form homodimers (same protein pairs) or heterodimers (different protein pairs) to increase the diversity and specificity of gene regulation.

Why Dimerization Matters:
Allows for more gene regulation options.
Alters how a TF regulates gene expression, like switching from activation to repression, depending on the TF partners.
Heterodimers can bind different DNA sequences and increase complexity in gene regulation.
Cooperative Binding → Two transcription factors can bind separately to nearby DNA sites, but they work together to stabilize each other’s binding and enhance gene transcription.

Example:
NFAT and AP1 in the IL-2 gene. Both TFs bind cooperatively to increase transcription.
Cooperative binding allows TFs to regulate genes in a more fine-tuned way, depending on the presence of multiple signals.
Big Picture Takeaways:
Repressors block transcription, while activators promote it, with TFs having specific DNA-binding motifs.
Dimerization allows transcription factors to combine and expand the variety of gene regulation possibilities.
Cooperative binding adds even more flexibility by letting unrelated TFs bind near each other and enhance transcription through their interactions.