Genetics and Evolution- Molecular Basis of Inheritance - Repressible Operon

Slide 1

  • Genetics and Evolution: Molecular Basis of Inheritance
    • Repressible Operon

Slide 2

  • What is a repressible operon?
  • Definition: A group of genes that are controlled by a single promoter and operator region, and their expression can be repressed when a regulatory protein binds to the operator.

Slide 3

  • Components of a repressible operon:
    • Promoter: Initiates transcription of the genes
    • Operator: Regulatory DNA sequence that controls the accessibility of the promoter to RNA polymerase
    • Structural Genes: Code for enzymes or proteins with a specific function

Slide 4

  • Example of repressible operon: The trp Operon
  • Involved in the synthesis of tryptophan, an essential amino acid
  • When tryptophan is abundant, the operon gets repressed and the synthesis of tryptophan is reduced

Slide 5

  • Trp Operon:
    • Consists of five genes: trpE, trpD, trpC, trpB, trpA
    • Regulatory region contains promoter, operator, and regulatory gene

Slide 6

  • Regulation of trp Operon:
    • Regulatory protein Trp repressor binds to the operator when tryptophan is present in excess
    • Binding of Trp repressor blocks the RNA polymerase from transcribing the structural genes

Slide 7

  • Tryptophan Absent:
    • RNA polymerase can freely bind to the promoter and transcribe the structural genes
    • Enzymes for tryptophan synthesis are produced

Slide 8

  • Tryptophan Present:
    • Excess tryptophan acts as a corepressor
    • Tryptophan binds to the Trp repressor protein, causing it to bind to the operator
    • This inhibits RNA polymerase from transcribing the structural genes

Slide 9

  • Feedback Inhibition:
    • Tryptophan acts as a feedback inhibitor on the trp operon
    • When tryptophan levels are high, the operon is repressed to prevent unnecessary synthesis of tryptophan

Slide 10

  • Importance of Repressible Operons:
    • Allows cells to regulate gene expression based on the availability of certain substances
    • Efficient utilization of resources
    • Avoids unnecessary synthesis of biomolecules.

Slide 11

  • Mechanism of Repression:
    • Tryptophan acts as a regulator of the trp operon
    • When tryptophan is absent, the repressor protein is unable to bind to the operator region
    • RNA polymerase can freely transcribe the structural genes

Slide 12

  • Mechanism of Repression (contd.):
    • When tryptophan is present, it binds to the repressor protein
    • The repressor-operator complex is formed and prevents RNA polymerase from transcribing the structural genes

Slide 13

  • Anabolic Pathways:
    • The trp operon is an example of anabolic pathway regulation
    • Anabolic pathways are responsible for the synthesis of complex molecules from simpler precursors
    • Regulation is important to avoid unnecessary production of complex molecules

Slide 14

  • Negative Control:
    • Repressible operons function through negative control
    • It involves the binding of a repressor protein to the operator region
    • Binding inhibits the transcription process

Slide 15

  • Comparison with Inducible Operons:
    • Repressible and inducible operons are two types of gene regulation systems
    • Repressible operons are usually active and repression occurs as a response to excess product
    • Inducible operons are usually inactive, and transcription is induced in response to a specific molecule

Slide 16

  • TrpR Gene:
    • trpR is the regulatory gene in the trp operon
    • It codes for the Trp repressor protein that binds to the operator
    • Expression of trpR is constitutive to ensure the presence of repressor protein

Slide 17

  • Tryptophan Regulation in E. coli:
    • In E. coli, tryptophan biosynthesis is regulated by the trp operon
    • When tryptophan levels are low, the operon is active and tryptophan synthesis occurs
    • When tryptophan levels are high, the operon is repressed to avoid unnecessary synthesis

Slide 18

  • Importance of Tryptophan:
    • Tryptophan is an essential amino acid required for various cellular processes
    • It serves as a precursor for the synthesis of important biomolecules such as proteins and neurotransmitters

Slide 19

  • Regulation and Gene Expression:
    • Gene regulation is a fundamental process in molecular biology
    • It allows cells to adapt to different environmental conditions
    • Repressible operons play a crucial role in regulating gene expression

Slide 20

  • Conclusion:
    • The trp operon is a prime example of a repressible operon
    • Tryptophan acts as a corepressor and regulates the transcription of tryptophan synthesis genes
    • Repressible operons help cells conserve resources by producing specific proteins only when needed

Slide 21

  • Lac Operon: An Inducible Operon
    • Controls the metabolism of lactose in bacteria
    • Composed of the promoter, operator, regulatory gene, and structural genes

Slide 22

  • Regulation of the Lac Operon
    • Lac operon is usually inactive and needs to be activated by an inducer molecule, such as lactose
    • Lactose acts as an inducer by binding to the Lac repressor and preventing it from binding to the operator
    • RNA polymerase can then transcribe the structural genes for lactose metabolism

Slide 23

  • Components of the Lac Operon
    • Promoter: Where RNA polymerase binds to initiate transcription
    • Operator: Regulatory DNA sequence that controls access to the promoter
    • LacI gene: Codes for the Lac repressor protein
    • Structural genes: lacZ, lacY, and lacA, code for enzymes necessary for lactose metabolism

Slide 24

  • Lac Repressor Protein
    • LacI gene codes for the Lac repressor protein
    • In the absence of lactose, the repressor binds to the operator and inhibits transcription
    • When lactose is present, it binds to the repressor, inducing a conformational change and preventing it from binding to the operator

Slide 25

  • Induction of the Lac Operon
    • In the absence of lactose, Lac repressor binds to the operator, blocking transcription of the structural genes
    • When lactose is present, it binds to the Lac repressor, causing it to detach from the operator
    • RNA polymerase can now bind to the promoter and transcribe the genes

Slide 26

  • Positive Control of the Lac Operon
    • The Lac operon can also be positively controlled by the molecule cAMP
    • In the absence of glucose, cAMP levels increase and bind to the cAMP receptor protein (CRP)
    • CRP-cAMP complex enhances the binding of RNA polymerase to the promoter, leading to increased transcription

Slide 27

  • Examples of Inducible Operons
    • Besides the Lac operon, other examples of inducible operons include:
      • Arabinose operon: involved in the metabolism of arabinose
      • Tryptophanase operon: involved in the metabolism of tryptophanase

Slide 28

  • Importance of Inducible Operons
    • Inducible operons allow bacteria to respond to changes in their environment
    • They ensure the efficient utilization of available resources
    • By regulating gene expression, bacteria can adapt to various conditions and optimize their growth

Slide 29

  • Comparison of Repressible and Inducible Operons
    • Repressible operons are usually active and get repressed in response to excess product
    • Inducible operons are usually inactive and get induced in response to a specific molecule
    • Both types of operons allow gene expression to be regulated based on need and availability of specific substances

Slide 30

  • Summary
    • Repressible and inducible operons are essential in gene regulation
    • Repressible operons get repressed when the product is in excess
    • Inducible operons get induced in response to a specific molecule
    • Both types of operons ensure efficient utilization of resources and adaptation to changing environments