Lecture Notes: The trp Operon

Introduction to Tryptophan and the trp Operon

• Tryptophan is one of 20 amino acids needed for protein synthesis in cells.
• Genes responsible for tryptophan biosynthesis are grouped together, regulated by a single promoter.
• This gene cluster and its regulatory sequences are known as the trp operon.
• E. coli bacteria express trp operon genes when tryptophan availability is low.
• When tryptophan is plentiful, the expression of these genes is repressed.

Mechanism of trp Operon Expression and Repression

• Expression: Occurs when tryptophan levels are low, allowing for biosynthesis.
• Repression: High levels of tryptophan act as a corepressor, inhibiting gene expression.

Comparison with the lac Operon

• lac Operon: An inducible system where genes are turned off unless lactose is present.
  • Lactose (the inducer) prevents the repressor from binding to the operator, enabling transcription.
• trp Operon: A repressible system always expressed unless tryptophan is available.
  • Tryptophan (the corepressor) enables the repressor to bind to the operator, blocking transcription.

Inducible vs. Repressible Systems

• Inducible Systems:
  • External substance (inducer) interacts with the regulatory gene product (repressor).
  • Interaction prevents the repressor from binding to the operator, allowing transcription.
• Repressible Systems:
  • Internal substance (corepressor) interacts with the regulatory gene product.
  • Interaction allows the repressor to bind to the operator, inhibiting transcription.
• Both systems, despite having opposite effects, share a similar mechanism.

Conclusion

• E. coli contains various operons regulated under different metabolic conditions.
• Understanding the functionality of operons, both inducible and repressible, is crucial to grasping gene expression regulation in prokaryotes.

Textbook Reference

• 16.1 How Is Gene Expression Regulated in Prokaryotes? p. 332