Regulation of Gene Expression

Basics of Gene Regulation - Control Central

• Gene Regulation: Microbes precisely control gene expression to adapt to environmental changes, conserve energy, and ensure cellular functions. Essential for survival and virulence.
• Levels of Control:
  • Transcriptional: Most common; controls RNA synthesis.
  • Translational: Controls protein synthesis from mRNA.
  • Post-translational: Modifies protein activity after synthesis.
• Gene Types:
  • Constitutive (Housekeeping): Always expressed; essential functions.
  • Inducible/Repressible: Expression turned ON/OFF as needed.
• Key Elements:
  • DNA Sequences: Promoters (initiate transcription), Operators (repressor binding site), Enhancers (activator binding site).
  • Regulatory Proteins: Activators (↑ transcription), Repressors (↓ transcription).

⭐ Transcriptional control is the most energy-efficient and therefore the most common mode of gene regulation in prokaryotes.

The Operon Model - Switch On/Off

• Operon: Functional DNA unit: promoter, operator, structural genes.
• Lac Operon (Inducible - lactose catabolism)
  • Genes: lacZ (β-galactosidase), lacY (permease), lacA (transacetylase).
  • Negative Control: Repressor (LacI) binds operator (lactose absent). Inducer: Allolactose.
  • Positive Control: ↓Glucose → ↑cAMP → CAP-cAMP complex binds promoter → ↑transcription.
  • 📌 Mnemonic: LAG: Lac Absent, Gluc present=OFF; LAP: Lac Absent, Gluc Absent=OFF (CAP ready); LPG: Lac Present, Gluc present=Low expr; LPA: Lac Present, Gluc Absent=ON.

  ⭐ Allolactose, an isomer of lactose, is the actual inducer for the lac operon, not lactose itself.

• Trp Operon (Repressible - tryptophan synthesis)
  • Genes: trpE, D, C, B, A.
  • Negative Control: Repressor inactive. Corepressor: Tryptophan. High Trp → Repressor active → Operon OFF.
  • Attenuation: Fine-tuning via leader sequence (trpL).
    • Low Trp: Ribosome stalls → anti-termination loop → transcription ON.
    • High Trp: Ribosome proceeds → termination loop → transcription OFF.

![Trp Operon Attenuation Mechanism](https://ylbwdadhbcjolwylidja.supabase.co/storage/v1/object/public/notes/L1/Microbiology_Microbial_Genetics_Regulation_of_Gene_Expression/2b82d0b9-b573-4584-9ff5-2959ff9722b8.svg)

Broader Regulatory Networks - Smart Systems

• Catabolite Repression (Glucose Effect): Global regulation prioritizing glucose.
  • Low Glucose: ↑ cAMP → cAMP binds Catabolite Activator Protein (CAP/CRP) → CAP-cAMP complex activates operons for alternative sugars (e.g., lac).
  • High Glucose: ↓ cAMP → CAP inactive → repression of these operons.

• Two-Component Regulatory Systems (TCS): Key for signal transduction.
  • Sensor Kinase (SK): Transmembrane histidine kinase; detects external signal, autophosphorylates.
  • Response Regulator (RR): Cytoplasmic; receives phosphate from SK, typically altering gene expression.
• Quorum Sensing (QS): Cell-density dependent gene regulation.
  • Bacteria produce & release autoinducers (AIs).
  • High cell density → ↑ AIs → threshold reached → coordinated gene expression.
  • Autoinducers:
    • Gram-negative: Acyl-Homoserine Lactones (AHLs).
    • Gram-positive: Autoinducing Peptides (AIPs).
  • Roles: Biofilm formation, virulence factor expression.

  ⭐ Many pathogenic bacteria utilize quorum sensing to coordinate the expression of virulence factors, only launching an attack when sufficient numbers are present.

High‑Yield Points - ⚡ Biggest Takeaways

• Operons (e.g., lac, trp) are fundamental for prokaryotic gene regulation.
• Lac operon: Inducible by lactose; CAP-cAMP mediates positive control (catabolite repression), repressor mediates negative control.
• Trp operon: Repressible by tryptophan; also features attenuation for fine-tuning.
• Sigma factors direct RNA polymerase to specific promoters for transcription initiation.
• Two-component regulatory systems allow bacteria to sense and adapt to environmental stimuli.
• Quorum sensing enables cell density-dependent gene expression and group behaviors.