Q29. A DNA-binding motif is
Correct Answer: (B) Helix-turn-helix
The helix-turn-helix (HTH) motif is the classic DNA-binding motif in transcription factors, where two alpha helices connected by a short turn allow one helix to fit into the DNA major groove for sequence-specific binding.
Option Analysis
(A) Helix-loop-helix
Helix-loop-helix (HLH) is a protein dimerization and DNA-binding domain found in eukaryotic transcription factors like MyoD, consisting of two helices separated by a flexible loop that promotes dimerization before DNA contact. It binds DNA via an adjacent basic region (bHLH), but the core motif emphasizes dimerization over direct binding.
(B) Helix-turn-helix ✓
This motif features two alpha helices linked by a tight beta-turn; the recognition helix inserts into the DNA major groove for base-specific interactions, common in prokaryotic repressors/activators like lambda repressor. It defines a true DNA-binding motif.
(C) Helical wheel
A helical wheel is a 2D projection diagram used to visualize amphipathic alpha helices in coiled-coil or leucine zipper proteins, predicting hydrophobic interactions but not a structural DNA-binding motif itself.
(D) Loop-helix-loop
No standard DNA-binding motif matches this exactly; it resembles parts of RNA-binding motifs (e.g., in tRNA synthetases) or misnomers, but lacks recognition as a DNA-specific binder unlike HTH or HLH.
The DNA-binding motif helix-turn-helix stands out as a fundamental structure in molecular biology, enabling transcription factors to recognize specific DNA sequences crucial for gene regulation—essential knowledge for GATE Life Sciences aspirants.
What is a DNA-Binding Motif?
DNA-binding motifs are conserved protein domains that interact with DNA, typically via the major groove, to control transcription. Helix-turn-helix exemplifies this by positioning a recognition helix for precise base contacts.
Key Types of DNA-Binding Motifs
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Helix-Turn-Helix (HTH): Two α-helices joined by a β-turn; recognition helix binds major groove. Prevalent in bacterial regulators.
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Helix-Loop-Helix (HLH): Two helices linked by a loop for dimerization; basic region binds DNA. Key in eukaryotic development.
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Others: Zinc finger (Zn-coordinated loops), leucine zipper (coiled-coil dimerization).
| Motif | Structure | Primary Function | Examples |
|---|---|---|---|
| Helix-Turn-Helix | 2 helices + turn | Direct DNA binding | Lambda repressor |
| Helix-Loop-Helix | 2 helices + loop | Dimerization + binding | MyoD, E47 |
| Helical Wheel | Projection model | Amphipathicity visualization | Leucine zippers |
Biological Significance
These motifs regulate gene expression; HTH dominates prokaryotes, while HLH drives multicellular processes like myogenesis. Mutations disrupt development or cause cancer.
