Regulation of Gene Expression in Prokaryotic Cells

Basic Concepts

• Cells must control the transcription process to synthesize proteins, referred to as regulation of gene expression.
• Prokaryotic cells (such as bacteria) use DNA-binding proteins to regulate gene expression, including:
  • Repressor: Blocks transcription.
  • Activator: Promotes transcription.

Prokaryotic Gene Regulation Model: Operon

• An operon is a DNA unit including:
  • Regulatory region: Contains control sites (promoter and operator).
  • Coding region: Encodes specific polypeptides or proteins.

Function of Sites within the Operon

• Promoter: Where RNA polymerase binds to initiate transcription.
• Operator: Where the repressor binds to block transcription.
• Regulatory gene: Encodes for a repressor or activator near the operon.

Example: Lac Operon in E.coli

• Lac operon: Used by E.coli to regulate lactose metabolism.

• When glucose is high:

  • RNA polymerase binds to the promoter of the regulatory gene, synthesizing the repressor protein.
  • The repressor binds to the operator, blocking transcription of genes encoding enzymes for lactose breakdown.
  • The cell uses glucose as the energy source, thus not needing to metabolize lactose.

• When glucose is low:

  • The concentration of cyclic AMP (cAMP) increases, stimulating the CAP protein.
  • Lactose is converted to allolactose, inactivating the repressor by binding to it.
  • When the repressor leaves the operator, CAP binds to the cap site, activating the promoter.
  • RNA polymerase binds to the promoter, initiating transcription of lactose breakdown genes to produce glucose and ATP.

Conclusion

• Glucose and lactose concentrations, along with cAMP concentration, are key regulatory factors in the operon.
• Regulation of gene expression helps the cell efficiently utilize available energy sources.