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.