Practice question

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. Only when intracellular tryptophan pool rises does allosteric conformational shift increase operator affinity hundredfold, permitting efficient repression, demonstrating apo-form remains inactive permitting biosynthesis specifically during starvation conditions.