TrpR

TrpR, the tryptophan repressor, is a bacterial transcription regulator that controls the expression of the tryptophan biosynthesis genes. It is encoded by the trpR gene and functions as part of a regulatory system that maintains intracellular tryptophan levels by repressing the trp operon when tryptophan is abundant. In Escherichia coli, the trpEDCBA operon encodes the enzymes for tryptophan synthesis, and its transcription is inhibited by TrpR in the presence of tryptophan.

The regulatory mechanism is allosteric. In low tryptophan conditions, TrpR has low DNA-binding affinity and the operon is transcribed, allowing synthesis of tryptophan. When intracellular tryptophan increases, it binds to TrpR as a corepressor, promoting a conformational change that enhances the protein’s affinity for the operator sequence (O_trp) near the trp promoter. The TrpR-tryptophan complex binds the operator as a dimer and obstructs RNA polymerase binding or progression, thereby repressing transcription of the operon.

Structural features include a dimeric architecture with an N-terminal DNA-binding domain that recognizes the operator and a C-terminal regulatory domain that binds tryptophan. The system exemplifies a classic repressible operon, where end-product feedback inhibits its own synthesis. TrpR homologs and related regulators are found across bacteria, with variations in operator sequence and regulatory strength contributing to species-specific control of tryptophan biosynthesis.