Lecture Notes: The Lac Operon and Gene Expression Regulation

Overview of Gene Expression

• Genomic DNA of Bacteria:
  • Contains many genes.
  • Genes undergo transcription and translation to form proteins.
  • Both processes are energy-intensive.
• Constitutive Expression:
  • Genes required for homeostasis are continuously expressed.
• Regulation by Extracellular Signals:
  • Signals are usually molecules in the growth medium.
  • Activators: Positively regulate gene expression.
  • Repressors: Negatively regulate gene expression.
  • Both are DNA-binding proteins.

DNA Binding Sites

• Operator: DNA sequence where a repressor binds.
• Activator Binding Site: DNA sequence where an activator binds.

Lac Operon Structure

• Operon: Functional unit of DNA with a cluster of genes under a single promoter.
• Genes in Lac Operon:
  • lacZ: Encodes β-galactosidase enzyme (breaks lactose into glucose and galactose).
  • lacY: Encodes lactose permease enzyme (transports lactose into the cell).
  • lacA: Encodes thiogalactoside transacetylase enzyme (removes toxic thiogalactosides).
• lacI: Encodes a repressor that binds the operator and represses the lac operon.

Functioning of Lac Operon

• In Absence of Lactose:
  • Operator occupied by lac repressor, shutting down lac operon genes (basal level transcription).
  • Repressor may occasionally fall off, allowing minimal transcription.
  • Some lactose permease, β-galactosidase, and thiogalactoside transacetylase are present.
• In Presence of Lactose:
  • Lactose enters cell via lactose permease.
  • Lactose binds to repressor, releasing it from the operator.
  • RNA polymerase binds promoter, initiating transcription of lac genes.
  • Leads to increased levels of enzymes facilitating more lactose entry and breakdown.

Role of CAP Protein in Lac Operon

• CAP: Catabolic Activator Protein, also known as cAMP Receptor Protein.
• Mechanism:
  • Low glucose increases cAMP levels.
  • cAMP binds CAP, activating it.
  • cAMP-CAP complex binds to CAP site, interacting with RNA polymerase, increasing transcription rate of lac genes.