Practice question

Tryptophan acts as co-repressor, small-molecule partner that completes repressor functionality, not inducer. Aporepressor alone cannot efficiently block transcription because DNA-binding domains oriented suboptimally. Binding of two L-tryptophan molecules per dimer into hydrophobic core supplies extensive hydrophobic and hydrogen-bond interactions that reposition helix-turn-helix motifs enabling direct major groove contacts with operator palindromic bases and phosphate backbone. Rising intracellular tryptophan pool thus directly signals metabolic sufficiency, converting inactive aporepressor into active repressor complex capable of shutting further synthesis. Mechanism contrasts with allolactose that inactivates LacI repressor in lac catabolic system upon substrate availability.