Transcription Factors

--- # **TRANSCRIPTION FACTORS AND RNA POLYMERASE IN EUKARYOTES AND PROKARYOTES** ## **1. RNA POLYMERASE IN PROKARYOTES VS. EUKARYOTES** ### **A. Prokaryotic RNA Polymerase** - In **prokaryotic organisms** (such as bacteria), transcription is carried out by a **single RNA polymerase enzyme**. - This **RNA polymerase does not require additional transcription factors** to initiate transcription. - Instead, the enzyme recognizes specific **promoter sequences** with the help of a **sigma (σ) factor** that enables the enzyme to locate the transcription start site. - Once transcription begins, the **σ factor is released**, allowing the core enzyme to elongate the RNA transcript. ### **B. Eukaryotic RNA Polymerase** - **Eukaryotic transcription is more complex** and requires **three distinct RNA polymerases**:**RNA Polymerase I** – Synthesizes rRNA (except 5S rRNA). - **RNA Polymerase II** – Synthesizes mRNA, some small nuclear RNAs (snRNAs), and microRNAs. - **RNA Polymerase III** – Synthesizes tRNA, 5S rRNA, and other small RNAs. - RNA Polymerase II, which transcribes **mRNA**, **requires multiple transcription factors** to recognize and bind to promoter regions, initiate transcription, and regulate gene expression. ## **2. TRANSCRIPTION FACTORS IN EUKARYOTES** ### **A. Definition and Role** - **Transcription factors (TFs)** are regulatory proteins that **assist RNA polymerase in initiating transcription** by recognizing and binding to promoter regions. - They help in **unwinding DNA, stabilizing RNA polymerase binding**, and **regulating gene expression**. ### **B. Transcription Factors for RNA Polymerase II** - **RNA Polymerase II requires six essential transcription factors**, which are collectively called **General Transcription Factors (GTFs)**. - These factors are necessary for the **initial stages of transcription** and help in:**Recruiting RNA Polymerase II to the promoter region**. - **Unwinding DNA at the transcription start site**. - **Stabilizing the transcription complex**. - **Regulating the initiation process**. ### **C. The Six General Transcription Factors for RNA Polymerase II** Each transcription factor has **specific roles in the formation of the transcription initiation complex**: - **TFIID (TATA-Binding Protein, TBP)****Structure & Function**:**Contains TBP (TATA-Binding Protein)**, a monomeric protein that recognizes the **TATA box**, a conserved promoter sequence. - **TBP binding to the TATA box is the first step in transcription initiation**. - TFIID also contains **TAFs (TBP-associated factors)**, which help **regulate transcription** and provide specificity to different promoters. - **Key Role**: Initiates transcription by binding to the TATA box in the **regulatory region** of the gene. - **TFIIA****Structure & Function**:A **multimeric protein** composed of three subunits. - It **stabilizes the binding of TFIID (TBP)** to the TATA box. - Enhances **RNA polymerase II recruitment** to the promoter. - **Key Role**: Ensures **stability** of the transcription initiation complex. - **TFIIB****Structure & Function**:A **monomeric protein** that **binds to TFIID (TBP)**. - Helps **position RNA Polymerase II** correctly at the transcription start site. - **Recruits TFIIF and RNA Polymerase II** to form the pre-initiation complex. - **Key Role**: **Bridges** the interaction between **TBP and RNA Polymerase II**, ensuring accurate transcription start site selection. - **TFIIF****Structure & Function**:A **multimeric protein** with two subunits. - Binds tightly to **RNA Polymerase II**, preventing it from **binding non-specifically to DNA**. - Helps RNA Polymerase II **bind specifically to the correct promoter**. - **Key Role**: **Prevents random DNA binding** and **stabilizes RNA Polymerase II** during initiation. - **TFIIE****Structure & Function**:A **multimeric protein** with two subunits. - **Recruits TFIIH**, an essential transcription factor with enzymatic activity. - Plays a role in **DNA unwinding**, allowing the transcription machinery to access the DNA template. - **Key Role**: **Facilitates DNA opening** by recruiting **TFIIH**. - **TFIIH****Structure & Function**:A **multimeric protein** with 12 subunits. - Contains **helicase activity**, which **unwinds DNA at the transcription start site**. - **Phosphorylates RNA Polymerase II**, activating it for elongation. - Involved in **nucleotide excision repair (NER)**, a DNA repair mechanism that corrects transcription errors. - **Key Role**:**Unwinds DNA** to allow transcription initiation. - **Phosphorylates RNA Polymerase II** to trigger the **elongation phase**. - Plays a role in **DNA repair**. ## **3. FORMATION OF THE TRANSCRIPTION INITIATION COMPLEX** The process of transcription initiation follows these steps: - **TFIID (TBP) binds to the TATA box**, marking the transcription start site. - **TFIIA stabilizes TFIID binding**. - **TFIIB bridges TBP and RNA Polymerase II**, recruiting the enzyme to t