Concerning the molecular mechanisms of eukaryotic transcription and its regulation, all of the following are true EXCEPT:

Explanation

## Eukaryotic RNA Polymerase Specificity and Transcription Initiation ### RNA Polymerase II and General Transcription Factors **Key Point:** RNA polymerase II (which transcribes all protein-coding genes and most non-coding RNAs) is absolutely dependent on general transcription factors (GTFs): - **TFIID** — contains TBP (TATA-binding protein) and TAFs (TBP-associated factors) - **TFIIB** — facilitates polymerase positioning and promoter escape - **TFIIE, TFIIF, TFIIH** — facilitate open complex formation and phosphorylation of the C-terminal domain (CTD) of Pol II ### TATA Box Recognition **High-Yield:** The TATA box (consensus sequence TATAAA) is located ~25–30 bp upstream of the transcription start site (+1). TBP (a component of TFIID) recognizes and binds the TATA box, bending the DNA and nucleating the pre-initiation complex (PIC). ### Mediator Complex **Key Point:** The Mediator complex is a large (~30 subunit) coactivator that: - Bridges enhancer-bound transcription factors and RNA Pol II - Facilitates long-range chromatin looping - Integrates signals from multiple regulatory proteins - Is essential for gene activation in response to cellular signals ### Why Option 3 Is Incorrect: Different Polymerases, Different GTFs **Warning:** RNA polymerase I, II, and III are structurally distinct and have **different promoter architectures and different sets of transcription factors**. | Feature | RNA Pol I | RNA Pol II | RNA Pol III | | --- | --- | --- | --- | | **Transcribes** | 18S, 5.8S, 28S rRNA | mRNA, miRNA, lncRNA, snRNA | tRNA, 5S rRNA, U6 snRNA | | **Promoter elements** | Core element, upstream control element | TATA box, CAAT box, GC box | Internal promoter (A/B boxes) or external promoter | | **GTF requirement** | SL1 (contains TBP) | TFIID, TFIIB, TFIIE, TFIIF, TFIIH | TFIIIB, TFIIIC | | **TBP-containing complex** | SL1 | TFIID | TFIIIB | **Mnemonic:** **POL-I-USES-SL1, POL-II-USES-TFIID, POL-III-USES-TFIIIB** — Each polymerase has its own dedicated transcription factor set. ### Detailed Comparison of GTFs **RNA Polymerase I:** - Requires **SL1** (in humans; equivalent to UBF in other organisms) - SL1 contains TBP and associated factors - Promoter is upstream of the gene (no TATA box) **RNA Polymerase II:** - Requires **TFIID, TFIIB, TFIIE, TFIIF, TFIIH** - TFIID recognizes TATA box via TBP - Requires Mediator for enhancer-driven activation **RNA Polymerase III:** - Requires **TFIIIB and TFIIIC** - TFIIIB contains TBP but recognizes internal promoter elements (A and B boxes) - Does NOT require Mediator for activation **Clinical Pearl:** Mutations in TFIID or other Pol II GTFs can cause developmental disorders (e.g., some cases of developmental delay). Conversely, mutations in Pol I or Pol III GTFs are rarer but affect ribosome and tRNA synthesis, leading to ribosomopathies.