Which of the following is NOT a structural DNA-binding motif commonly found in transcription factors?

Introduction / Context:
DNA-binding proteins often use recurring structural motifs to contact specific sequences in promoters and enhancers. Recognizing these motifs helps identify likely transcription factors from domain annotations.

Given Data / Assumptions:

• We are comparing structural motifs, not entire multiprotein complexes.
• Motifs are short, recurring 3D arrangements such as helix–turn–helix or zinc finger.

Concept / Approach:
Helix–turn–helix, zinc finger, bZIP, and helix–loop–helix are classical DNA-binding motifs. TFIID, however, is a large multi-subunit general transcription factor complex (containing TBP and TAFs), not a single structural motif.

Step-by-Step Solution:

Verification / Alternative check:
Protein domain databases categorize helix–turn–helix, zinc finger, bZIP, and HLH as motifs, while TFIID is cataloged as a complex assembling at core promoters to recruit RNA polymerase II.

Why Other Options Are Wrong:

• Helix–turn–helix: Classic motif in many bacterial repressors and eukaryotic factors.
• Zinc finger: Cys2His2 and other variants bind DNA via coordinated zinc ions.
• bZIP: Dimeric motif with basic region for DNA contact and leucine zipper for dimerization.
• Helix–loop–helix: Found in developmental regulators; facilitates dimerization and DNA binding.

Common Pitfalls:
Confusing a multiprotein complex name (TFIID) with a structural motif, or assuming all transcription-related terms are motifs.

Final Answer:
TFIID.