The Trp Operon in E. coli

Overview

• The trp operon is a cluster of genes in E. coli responsible for the synthesis of tryptophan, an essential amino acid.
• Includes five structural genes: trpE, trpD, trpC, trpB, and trpA.
• These genes encode enzymes required for tryptophan synthesis.
• Tryptophan is crucial for protein synthesis and must be present continuously for E. coli survival.

Operon Structure

• Structural Genes: Share a common promoter and operator.
• Regulatory Gene: trpR, located upstream, encodes an inactive repressor.

Operon Function

• Inactive Repressor: Cannot bind to the operator, allowing RNA polymerase to bind to the promoter and transcribe the genes (de-repressed state).
• Tryptophan Production: Occurs when the operon is de-repressed, enabling the synthesis of enzymes for tryptophan production.

Repression Mechanism

• Excess Tryptophan: Acts as a co-repressor, binding to the inactive repressor and changing its shape to an active form.
• Active Repressor: Binds to the operator, preventing RNA polymerase from transcribing the structural genes, thus repressing the operon.
• Energy Conservation: Limits unnecessary tryptophan production when it is abundant, conserving cellular resources.

Conclusion

• The trp operon is a repressible operon, naturally expressed unless excess tryptophan is present to activate the repressor.
• De-repressed State: Occurs when tryptophan is scarce, allowing gene transcription and tryptophan production.
• Repressed State: Occurs when tryptophan is abundant, activating the repressor to limit gene transcription and additional tryptophan production.

Additional Resources

• For general operon understanding, see videos on operon basics.
• For learning about other operons, see videos on the lac operon.