Eukaryotic transcription initiation: Which general transcription factor complex binds first at the TATA box?
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ATranscription factor IIA (TFIIA)
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BTranscription factor IIB (TFIIB)
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CTranscription factor IID (TFIID with TBP)
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DAll of these
Answer
Correct Answer: Transcription factor IID (TFIID with TBP)
Explanation
Introduction / Context:Accurate transcription by RNA polymerase II requires assembly of general transcription factors at promoters. At many TATA-containing promoters, a characteristic order of assembly begins with TFIID recognizing the TATA box.
Given Data / Assumptions:
- Promoter contains a TATA element.
- General transcription factors include TFIID (TBP + TAFs), TFIIA, TFIIB, and others.
- The question asks which factor binds first.
Concept / Approach:TFIID contains TBP (TATA-binding protein), which bends DNA on binding the TATA sequence, serving as a platform for subsequent factors (TFIIA, TFIIB) and recruitment of RNA polymerase II with TFIIF, followed by TFIIE and TFIIH.
Step-by-Step Solution:
Identify the TATA motif as a TBP target.Recall TFIID = TBP + TAFs; TBP anchors to TATA.Subsequent factors stabilize and position Pol II.Choose TFIID as the first binder.Verification / Alternative check:In vitro reconstitution shows TBP/TFIID binding is the earliest and required step for many TATA promoters.
Why Other Options Are Wrong:
- TFIIA and TFIIB bind after TFIID to stabilize the complex and help Pol II positioning.
- “All of these” misstates the ordered assembly pathway.
Common Pitfalls:Assuming all promoters assemble identically; TATA-less promoters can recruit via other core elements, but TBP/TFIID is still central.
Final Answer:Transcription factor IID (TFIID with TBP)