Trp operon

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Trpoperon

Trp operon organization across three different bacterial species

Tryptophan biosynthesis (en)

Trp repressor dimer and operator

Trp operon attenuation

Trp operon is a cluster of genes responsible for the synthesis of tryptophan in bacteria, particularly in Escherichia coli (E. coli). The operon is an example of a repressible system in prokaryotic gene regulation, where the presence of tryptophan inhibits the production of enzymes involved in its own synthesis. This article delves into the structure, function, and regulation of the Trp operon, highlighting its significance in the broader context of genetic regulation and molecular biology.

Structure

The Trp operon consists of a promoter region (P), an operator region (O), a leader peptide region (L), and five structural genes (trpE, trpD, trpC, trpB, and trpA) that encode enzymes for tryptophan synthesis. The promoter region is where RNA polymerase binds to initiate transcription, and the operator is a sequence that can be bound by a repressor protein to inhibit transcription. The leader peptide region contains a short gene, trpL, which plays a role in the attenuation mechanism of the operon's regulation.

Function

The primary function of the Trp operon is to produce the enzymes necessary for the biosynthesis of tryptophan, an essential amino acid. The operon's structural genes encode for the components of a multi-enzyme complex that catalyzes the conversion of chorismate to tryptophan through several biochemical steps.

Regulation

The Trp operon is regulated by two main mechanisms: repression and attenuation.

Repression

In the presence of tryptophan, the Trp repressor protein binds to tryptophan, forming a tryptophan-repressor complex. This complex then binds to the operator region of the operon, preventing RNA polymerase from transcribing the structural genes. This feedback inhibition mechanism ensures that tryptophan is synthesized only when its levels are low in the cell.

Attenuation

Attenuation is a mechanism that further fine-tunes the regulation of the Trp operon by controlling the termination of transcription at the leader peptide region. The leader peptide contains sequences that can form stem-loop structures in the mRNA being transcribed. The formation of these structures depends on the concentration of tryptophan and indirectly on the speed of the ribosome during translation of the leader peptide. High levels of tryptophan increase the likelihood of transcription termination before RNA polymerase reaches the structural genes, thus reducing tryptophan synthesis.

Significance

The Trp operon serves as a classic model for understanding gene regulation in prokaryotes. It illustrates how cells can efficiently respond to nutritional changes and conserve resources by regulating enzyme production at the genetic level. The concepts of repressor proteins, operons, and feedback inhibition are fundamental to the fields of molecular biology and genetics.

See Also

• Gene expression
• Operon
• Prokaryotic transcription
• Regulation of gene expression
• Tryptophan