EM

Gene Regulation and Operons

DNA and Transcription Basics

  • Regulatory Sequences: Sites for regulatory proteins; influence transcription rate. Found in multiple locations.
    • Promoter: RNA polymerase binding site; signals start of transcription.
    • Terminator: Signals end of transcription.

mRNA Structure and Components

  • mRNA: Carries genetic information from DNA for protein synthesis.
    • Ribosomal Binding Site: Site where ribosomes bind; translation starts here.
    • Start Codon: Specifies the first amino acid in the polypeptide (e.g., formylmethionine in bacteria, methionine in eukaryotes).
    • Codons: Sequences of three nucleotides that specify amino acids; determine amino acid sequence in polypeptide.
    • Stop Codon: Signals end of polypeptide synthesis.
    • Polycistronic mRNA: Found in bacteria; encodes multiple polypeptides.

Gene Regulation Mechanisms

  • Repressor Proteins: Bind DNA to inhibit transcription (negative control).
  • Activator Proteins: Bind DNA to promote transcription (positive control).
  • Inducible Genes: Normally “off” but can be activated under specific conditions.
  • Repressible Genes: Normally “on” but can be suppressed under certain conditions.

The Lac Operon

  • Inducible System Example: The lac operon transcribes only when lactose is present.
  • Components:
    • lac Regulatory Gene: Produces the lac repressor.
    • Promoter (lacP): Site for RNA polymerase binding.
    • Operator (lacO): Where the lac repressor binds.
    • Structural Genes (lacZ, lacY, lacA): Encode necessary proteins for lactose metabolism.
  • Negative Control Mechanism:
    • No Lactose: lac repressor binds to operator, inhibiting transcription.
    • With Lactose: Allolactose binds to repressor, causing conformational change, preventing repressor from inhibiting transcription.

Jacob and Monod's Research

  • Identified mutants related to operon functions (e.g., P-, Oc, I-, Is).

CAP (Catabolite Activator Protein) Mechanism

  • Positive Control:
    • High cAMP levels enhance RNA polymerase binding, increasing transcription rate.
    • Low cAMP levels decrease transcription due to absence of CAP binding.
  • Transcription Rates:
    • Lactose, No Glucose: High cAMP; transcription is high.
    • No Lactose or Glucose: High cAMP; transcription is low.
    • Lactose and Glucose: Low cAMP; transcription is very low due to repressor activity.

The Tryptophan (Trp) Operon

  • Repressible System Example: Stops transcription when tryptophan is abundant.
  • Attenuation (only in prokaryotes): Simultaneous transcription and translation affect gene expression.
    • Low Tryptophan: Ribosome stalls at Trp codons, preventing termination and allowing transcription to continue.
    • High Tryptophan: Ribosome completes translation, forming a 3-4 stem-loop that terminates transcription.

Key Concepts of Attenuation

  • Coupled transcription and translation allow regulation based on tryptophan levels.
    • Low Levels: Region 2 bonds with region 3 only, avoiding termination.
    • High Levels: Region 3 bonds with region 4, leading to transcription termination.