In absence of tryptophan, trp repressor is
In absence of intracellular tryptophan, TrpR aporepressor exists predominantly in inactive conformation because tryptophan binding pockets remain empty leaving DNA-binding domains improperly aligned for precise operator groove insertion and base-specific contacts. Dissociation constant for specific operator DNA remains high micromolar range, fractional occupancy negligible under physiological repressor concentration. RNA polymerase holoenzyme therefore freely accesses -35 and -10 promoter elements, forms open complex, and initiates transcription of leader region and downstream structural genes
Ref: Pearson Trp Operon Explained – aporepressor inactive without tryptophan, operon remains derepressed during starvation.
