Inhibitors of Transcription

# **Inhibitors of Transcription** Transcription inhibitors are compounds that interfere with **RNA synthesis** by targeting RNA polymerase. Some of these inhibitors are used as **antibiotics**, while others are natural toxins. ### **1. Rifamycin (Rifampin) – Inhibitor of Prokaryotic Transcription** - **Produced by**: *Amycolatopsis rifamycinica* (a bacterial species). - **Targets**: **Prokaryotic RNA polymerase**. - **Mechanism of Action**:Binds to the **β-subunit of RNA polymerase**. - Prevents **elongation** of the mRNA strand. - Inhibits transcription initiation. - **Clinical Use**:Used to treat **Mycobacterium tuberculosis** (causes **tuberculosis**). - Effective against other bacterial infections. ### **2. α-Amanitin – Inhibitor of Eukaryotic Transcription** - **Produced by**: *Amanita phalloides* (death cap mushroom). - **Targets**: **Eukaryotic RNA polymerase II**. - **Mechanism of Action**:**Strongly inhibits RNA polymerase II**, which is responsible for synthesizing **mRNA**. - **Moderate effect** on RNA polymerase III (tRNA synthesis). - **No effect** on RNA polymerase I (rRNA synthesis). - **Effects**:Leads to **severe hepatotoxicity (liver damage)**. - Symptoms include **nausea, vomiting, diarrhea**, and eventually **organ failure**. # **Differences Between Transcription in Prokaryotes and Eukaryotes** # **Post-Transcriptional Modifications in Eukaryotic mRNA** - **5’ Capping**:Addition of a **7-methylguanosine cap** at the 5’ end. - Protects mRNA from **degradation** and helps in **ribosome binding**. - **Polyadenylation**:Addition of a **poly-A tail** at the 3’ end (about 200 adenine residues). - Helps in **stability and nuclear export** of mRNA. - **Splicing**:Removal of **introns** (non-coding sequences). - Exons (coding regions) are joined together to form **mature mRNA**. ### **Summary** - **Rifamycin** inhibits prokaryotic transcription by binding to **RNA polymerase β-subunit**. - **α-Amanitin** inhibits eukaryotic transcription by targeting **RNA polymerase II**. - **Key differences** exist between transcription in prokaryotes and eukaryotes, including location, mRNA processing, regulation, and termination. - **Eukaryotic mRNA requires post-transcriptional modifications** before translation. # **Translation: Protein Biosynthesis** Translation is the **second step in gene expression**, following transcription. It is the process by which **ribosomes synthesize proteins** using mRNA as a template. ## **1. Location of Translation** - **Prokaryotes**: Occurs in **free ribosomes** in the **cytoplasm**. - **Eukaryotes**: Ribosomes can be **free in the cytoplasm** or **attached to the rough endoplasmic reticulum (RER)**, where **protein processing occurs**. ## **2. Stages of Translation** Protein synthesis occurs in **three major steps**: ### **(A) Initiation** - The goal is to bring together the **mRNA**, **tRNA carrying the first amino acid**, and the **ribosomal subunits**. - The **mRNA** contains a sequence of **codons** (triplets of nucleotides) that dictate which **amino acids** will be incorporated into the protein. - **tRNA molecules** carry amino acids to the ribosome. - The **ribosome assembles**, with:The **small ribosomal subunit** binding to the **mRNA**. - The **large ribosomal subunit** joining later to complete the ribosome. ### **(B) Elongation** - The ribosome moves along the **mRNA**, reading codons and adding **amino acids** in the correct sequence. - Each new amino acid is added to the growing protein chain by **peptide bond formation**. - **Three key sites** in the ribosome:**A (Aminoacyl) Site** – Binds **incoming tRNA** carrying an amino acid. - **P (Peptidyl) Site** – Holds **tRNA** with the growing **polypeptide chain**. - **E (Exit) Site** – Where **empty tRNA exits** after donating its amino acid. ### **(C) Termination** - Occurs when the ribosome reaches a **stop codon** (**UAA, UAG, UGA**) on the mRNA. - A **release factor** binds to the ribosome, causing it to **disassemble**, and the newly formed **protein is released**. ## **3. Role of tRNA in Translation** - **tRNA (Transfer RNA)** molecules bring **amino acids** from the **cytoplasm to the ribosome**. - tRNA has a **unique cloverleaf structure** with **73-93 nucleotides**. - **Four distinct regions (lobes)** are present:**Anticodon Loop** – Contains a sequence complementary to the **mRNA codon** to ensure correct amino acid pairing. - **Amino Acid Attachment Site** – Located at the **3' end**, where the correct **amino acid** binds. ## **4. Summary of Key Concepts** - **Translation is the process of protein synthesis** from mRNA. - Takes place in the **ribosomes** in both prokaryotes and eukaryotes. - Involves **three stages**: **Initiation, Elongation, and Termination**. - **tRNA molecules play a crucial role** by bringing amino acids to the ribosome and matching codons with their anticodons. ### **Aminoacylation of tRNA (tRNA Charging)** #### **1. Attachment of Amino Acids to t