Genetics of Bacteria

Gene Regulation and Its Evolutionary Advantage

• Why Bacteria Regulate Genes:

  • Gene regulation conserves energy and resources by producing proteins only when needed.

  • Adaptability improves survival in fluctuating environments.

  • Example: E. coli in the human digestive system adjusts to variations in nutrient availability.

  • Natural selection favors bacteria with efficient transcriptional regulation mechanisms, allowing them to thrive in nutrient-limited or unpredictable conditions.

Levels of Metabolic Regulation in Bacteria

Regulation of Enzyme Activity (Feedback Inhibition)

• Mechanism: The activity of pre-existing enzymes is modulated.

• Example:

  • Tryptophan produced via a metabolic pathway inhibits the first enzyme in the pathway when it accumulates.

  • This is a fast but temporary response, regulating metabolic flux.

Regulation of Enzyme Production (Transcriptional Control)

• Mechanism: Adjusts the expression of genes encoding metabolic enzymes.

• Example:

  • If tryptophan is abundant, E. coli represses transcription of genes needed for its synthesis.

  • This response takes LONGER than enzyme activity regulation but conserves energy over extended periods.

The Operon Model

• Definition: An operon is a cluster of functionally related genes under the control of a single promoter and regulatory system.

• Components:

  1. Promoter: DNA sequence where RNA polymerase binds to initiate transcription.

  2. Operator: A regulatory "switch" DNA segment that controls RNA polymerase’s access to the promoter.

  3. Structural Genes: Genes encoding proteins for a specific metabolic pathway.

  4. Regulatory Gene: Located outside the operon; encodes a repressor protein.

The trp Operon: A Repressible Operon

Operon Structure and Function

• Transcribed as a single polycistronic mRNA, ensuring coordinated regulation.

• Operon is typically "on" and actively transcribed unless repressed.

Repression Mechanism

1. Repressor Protein:

   • Encoded by the regulatory gene trpR, expressed continuously at low levels.

   • Synthesized in an inactive form, unable to bind the operator.

2. Corepressor (Tryptophan):

   • When tryptophan levels are high, it acts as a corepressor by binding to the repressor protein.

   • This allosteric interaction activates the repressor.

   • Active repressor binds to the operator, preventing RNA polymerase from transcribing the operon.

3. Reversibility:

   • When tryptophan levels drop, the corepressor dissociates from the repressor, inactivating it.

   • Transcription resumes, producing enzymes to synthesize tryptophan.

The lac Operon: An Inducible Operon

Operon Structure and Purpose

• Encodes enzymes involved in lactose metabolism:

  1. lacZ: Encodes β-galactosidase (breaks down lactose).

  2. lacY: Encodes permease (transports lactose into the cell).

  3. lacA: Encodes transacetylase (function less understood).

• Operon is typically "off" unless lactose is present.

Induction Mechanism

1. Repressor Protein:

   • Encoded by the lacI gene, located outside the operon.

   • Active by default, binds to the operator, blocking transcription.

2. Inducer (Allolactose):

   • A metabolite of lactose that binds to the repressor protein, inactivating it.

   • Inactive repressor dissociates from the operator, allowing transcription.

3. Enzyme Production:

   • RNA polymerase transcribes the operon, producing enzymes to metabolize lactose.

Dual Control of the lac Operon

• Negative Control:

  • Involves the repressor and inducer interaction.

• Positive Control (CAP-cAMP System):

  • When glucose is scarce, cyclic AMP (cAMP) levels rise.

    • cAMP binds to the catabolite activator protein (CAP), activating it.

      • CAP-cAMP complex binds to the promoter, enhancing RNA polymerase binding.

  • Lac operon transcription is stimulated only when lactose is present and glucose is scarce.

6. Differences Between Repressible and Inducible Operons

• Operons allow bacteria to:

  1. Efficiently coordinate gene expression for related metabolic processes.

  2. Conserve energy by producing proteins only when needed.

  3. Adapt to environmental changes rapidly, ensuring survival in diverse conditions.