Practice question

trpR regulatory gene, located separately near threonine biosynthesis locus distant from main operon, encodes 108-amino-acid TrpR aporepressor forming stable symmetric dimer containing helix-turn-helix DNA recognition motifs at C-termini. Aporepressor alone binds tryptophan operator DNA weakly because recognition helices are suboptimally oriented for major groove insertion. Binding of L-tryptophan in hydrophobic pockets between dimerization domains stabilizes compact conformation positioning reading heads correctly to recognize eighteen-base pair palindromic operator overlapping trp promoter region, thereby functioning as corepressor-dependent transcriptional repressor rather than biosynthetic enzyme or attenuator component itself directly.