Practice question

DNA looping in ara operon results from simultaneous binding of AraC dimer to distantly separated operator half-sites, bringing them into close physical proximity through protein-mediated bending. Looped complex imposes severe steric block on RNA polymerase binding site, prevents formation of transcription bubble, and constrains promoter DNA topology in configuration unfavorable for activation. This mechanical hindrance efficiently silences araBAD transcription in absence of inducing sugar, more effective than simple occupancy of single operator site, analogous to LacI repressor tetramer looping between O1 and auxiliary O2 or O3 sites enhancing repression thousandfold through increased local concentration of repressor near promoter region.