16.14 Gene Control in Prokaryotes

1. What is Gene Expression?

• Definition:
  • Gene Expression: The process by which the information encoded in a gene is used to direct the synthesis of a functional product, usually a protein.
• Stages of Gene Expression:
  1. Transcription: The DNA sequence of a gene is transcribed into messenger RNA (mRNA).
  2. Translation: The mRNA is translated by ribosomes into a specific protein.
• Illustration:
  • Figure 1: Diagram showing DNA → mRNA → Protein synthesis.

2. Selective Gene Expression

• Definition:
  • Not all genes in an organism’s genome are expressed simultaneously. Selective expression ensures that only specific genes are active in a particular cell type, allowing for cellular differentiation and specialization.
• Examples:
  • Melanin Production: The melanin gene is expressed in skin cells, resulting in pigment production, but remains inactive in heart muscle cells.
  • Hemoglobin Synthesis: Only erythrocytes (red blood cells) express hemoglobin genes, enabling oxygen transport.
• Significance:
  • Cellular Specialization: Allows diverse cell types to perform specialized functions despite having the same genetic material.
  • Energy Efficiency: Conserves energy by only producing necessary proteins.

3. Gene Control in Prokaryotes

• Prokaryotic gene regulation is typically organized around operons, allowing coordinated expression of genes with related functions.

Operons in Bacteria

• Definition:
  • An operon is a cluster of genes under the control of a single promoter and regulatory elements, enabling them to be transcribed together as a unit.
• Components of an Operon:
  • Promoter: A DNA sequence where RNA polymerase binds to initiate transcription.
  • Operator: A regulatory DNA sequence where repressor proteins can bind to block transcription.
  • Structural Genes: Genes that encode proteins with related functions, transcribed together.
• Example: The lac Operon in Escherichia coli
  • Function: Regulates the metabolism of lactose, allowing the bacterium to utilize lactose when glucose is not available.
  • Structural Genes:
    • lacZ: Encodes β-galactosidase, which breaks down lactose into glucose and galactose.
    • lacY: Encodes permease, facilitating lactose entry into the cell.
    • lacA: Encodes transacetylase, involved in lactose detoxification.
  • Regulatory Elements:
    • Promoter (P): Site for RNA polymerase binding.
    • Operator (O): Binding site for the lac repressor protein.
    • Regulator Gene (lacI): Located adjacent to the operon, encodes the repressor protein.

4. Structural and Regulatory Genes

• Structural Genes:
  • Definition: Genes that encode proteins performing specific functions within the cell, such as enzymes, structural proteins, or transporters.
  • Example: lacZ encodes β-galactosidase, an enzyme that hydrolyzes lactose.
• Regulatory Genes:
  • Definition: Genes that produce proteins regulating the expression of other genes, often acting as repressors or activators.
  • Example: lacI encodes the lac repressor protein, which inhibits the lac operon in the absence of lactose.
• Figure 2: Diagram differentiating structural genes and regulatory genes within an operon.

5. The lac Operon Mechanism in E. coli

• Understanding the lac operon provides insight into how bacteria regulate gene expression in response to environmental changes.

Function of the lac Operon

• Role: Controls the production of enzymes necessary for the metabolism of lactose, specifically β-galactosidase, permease, and transacetylase.
• Process: Allows E. coli to efficiently utilize lactose when glucose is scarce.

Components of the lac Operon

1. lacZ: Encodes β-galactosidase.
2. lacY: Encodes permease.
3. lacA: Encodes transacetylase.
4. Promoter (P): Site where RNA polymerase binds to initiate transcription.
5. Operator (O): Binding site for the repressor protein.

Regulatory Gene: lacI

• Function: Produces the lac repressor protein, which regulates the operon by binding to the operator.
• Location: Typically located adjacent to the lac operon but is transcribed independently.

Mechanism of Action

1. When No Lactose Is Present:
   • Repressor Binding: The lac repressor protein binds to the operator region.
   • Transcription Blocked: RNA polymerase cannot access the promoter, preventing transcription of lacZ, lacY, and lacA.
   • Result: No production of β-galactosidase, permease, or transacetylase, conserving cellular resources.
2. When Lactose Is Present:
   • Lactose Uptake: Lactose enters the cell via permease.
   • Allolactose Formation: Lactose is converted into allolactose, which acts as an inducer.
   • Repressor Modification: Allolactose binds to the repressor, causing a conformational change.
   • Repressor Detachment: The modified repressor can no longer bind to the operator.
   • Transcription Initiated: RNA polymerase binds to the promoter, transcribing the structural genes.
   • Result: Production of β-galactosidase, permease, and transacetylase, enabling lactose metabolism.

• Figure 3: Detailed mechanism of the lac operon in the presence and absence of lactose.

6. Types of Enzymes Based on Regulation

• Enzymes involved in metabolism can be categorized based on how their synthesis is regulated:

Inducible Enzymes

• Definition: Enzymes synthesized only in the presence of their specific substrate.
• Mechanism: The substrate acts as an inducer, activating gene expression.
• Example: β-galactosidase in the lac operon is an inducible enzyme; it is produced only when lactose is present.

Repressible Enzymes

• Definition: Enzymes that are typically produced continuously but can be inhibited when a specific effector molecule is present.
• Mechanism: The effector molecule binds to a repressor, enabling it to bind to the operator and block transcription.
• Example: Amino acid biosynthesis enzymes (e.g., tryptophan synthase) are repressible; excess tryptophan inhibits their synthesis.
• Table 1:

7. Key Terms

• β-Galactosidase: An enzyme that hydrolyzes lactose into glucose and galactose; encoded by the lacZ gene.
• Operon: A cluster of genes under the control of a single promoter and regulatory elements, allowing coordinated gene expression.
• Lac Repressor Protein: A regulatory protein encoded by the lacI gene that inhibits transcription of the lac operon in the absence of lactose.
• Inducible System: A gene regulation system activated by the presence of a specific inducer molecule.
• Repressible System: A gene regulation system inhibited by the presence of a specific corepressor molecule.

8. Example Application: Adaptive Response of the lac Operon

• The lac operon exemplifies how bacteria adapt to environmental changes to optimize resource utilization.

• Adaptive Advantage:
  • Energy Efficiency: E. coli only produces lactose-metabolizing enzymes when lactose is available, conserving energy and resources.
  • Rapid Response: Enables quick adaptation to fluctuations in available nutrients.
• Environmental Cue: Presence or absence of lactose in the environment triggers the operon’s regulatory mechanism.
• Figure 4: Adaptive response of the lac operon to varying lactose concentrations.

9. Comparison with Repressible Systems

Understanding different gene regulation systems highlights the versatility of cellular control mechanisms.

• Inducible Systems (e.g., lac operon):
  • Activated by: Presence of substrate (e.g., lactose).
  • Purpose: Enable utilization of available resources.
• Repressible Systems (e.g., trp operon):
  • Activated by: Absence of end product (e.g., tryptophan).
  • Purpose: Prevent overproduction and conserve resources when the end product is abundant.
• Key Differences: