operon Trp

Tryptophan Operon (trp Operon)

The tryptophan operon is a bacterial system regulating the synthesis of the amino acid tryptophan. It controls the expression of five structural genes involved in the tryptophan synthesis pathway.

Key Functions

The operon ensures that when tryptophan levels are high, energy is conserved by preventing the unnecessary production of tryptophan.

Genetic Structure

The trp operon is situated on the circular bacterial chromosome, with the regulatory gene trpR located approximately a quarter of the way around.

trpR Gene:

  • Encodes a dimeric repressor protein

  • Produced at a slow but continuous rate.

  • Repressor proteins diffuse through the cytoplasm and regulate gene expression.

Control Sequences of the trp Operon

  • Promoter: The binding site for RNA polymerase.

  • Operator: Overlaps the promoter and is the site where the repressor binds to regulate transcription.

  • Leader Gene (trpL): Contains an attenuator sequence which is crucial for fine-tuning gene expression based on tryptophan levels.

Transcription Mechanism

  • Low Tryptophan Levels:

    • RNA polymerase binds to the promoter and transcribes structural genes.

    • Structural genes encode enzymes for the tryptophan synthesis pathway, including:

      • trpE and trpD: Anthranilate synthase.

      • trpC: Indole glycerol phosphate synthase.

      • trpB and trpA: Tryptophan synthase.

  • High Tryptophan Levels:

    • Tryptophan binds to the trp repressor.

    • Repressor shape changes, allowing it to bind to the operator.

    • This binding blocks RNA polymerase and prevents the transcription of structural genes.

    • Tryptophan acts as a co-repressor, regulating gene expression.

Attenuation Mechanism

  • Low Tryptophan Levels:

    • Ribosomes pause when they reach two tryptophan codons in region one of leader mRNA, awaiting trp-charged tRNAs.

    • This pause allows regions 2 and 3 of the mRNA to form a stem-and-loop structure.

    • Ribosomes then continue translating structural genes, leading to increased tryptophan synthesis.

  • High Tryptophan Levels:

    • Abundant trp-charged tRNAs allow ribosomes to quickly translate region one.

    • This promotes pairing between region 3 and the newly synthesized region 4.

    • A stem-and-loop structure forms, terminating transcription.

    • Result: Structural genes are not transcribed, reducing tryptophan production and allowing its cellular concentration to decrease.