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Negative control of transcription

  1. Negative control of transcription: Repression • Name:- Shubham D Maurya • Roll No: 814 • Subject:- Microbiology Paper 3 (PUSMB-503)
  2. INTRODUCTION • The bacteria have genomes that contain thousands of different genes. • Most of these genes encode proteins, each with its own role in a process such as fuel metabolism, maintenance of cell structure, and defense against viruses. • Some of these proteins are needed routinely, while others are needed only under certain circumstances. • Thus, cells don't express all the genes in their genome all the time.
  3. • Expression of a gene (Transcription and translation) is a highly regulated process. • Regulation - "Ability of a cell to control or regulate what proteins it makes from its DNA" • Regulation of genes in prokaryotes ensures that a cell's resources are not wasted in making proteins that the cell does not need at that time. • Lot of gene regulation occurs at the level of transcription. • Positive regulation - Activators • Negative regulation – Repressors*
  4. • Gene regulation in prokaryotes can be explained with the help of the Operon model. • Bacterial genes - Operon • Operon - a cluster of genes under the control of a single promoter. • An operon comprises of structural genes, promotor and operator. • Regulatory sequences • Regulatory gene - regulatory proteins. • Operator - Negative regulatory site. Repressor binds here.
  5. Negative Control: Repression • Negative control of transcription is a control mechanism that prevents transcription. It includes repression and induction. • "The enzymes that catalyze the synthesis of a specific product are not made if the product is already present in the medium in sufficient amounts. • Example:- Amino Acid Arginine • The enzymes needed to synthesize the amino acid arginine are made only when arginine is absent from the culture medium. • Arginine - Co-repressor – prevents the synthesis of the enzymes – enzyme repression.
  6. Bacteria growing in a media without arginine Time Relative Increase 1. Cell Number 2. Total Protein 3. Arginine Biosynthesis Enzymes Arginine Added
  7. Mechanism of Repression Transcription of the arginine genes, which proceeds when the cell needs arginine. 1. Expression – polycistronic mRNA 2. Translation – Enzyme - synthesised arginine.
  8. When arginine is plentiful it acts as corepressor. Arginine binds to a specific repressor protein, the arginine repressor, present in the cell. Repressor binds to the operator, transcription is physically blocked because RNA polymerase can neither bind nor proceed. Hence, the polypeptides encoded by the genes in the operon cannot be synthesized. mRNA is polycistronic, all the polypeptides encoded by this mRNA will be repressed