Practice question

LacI repressor is a tetrameric helix-turn-helix protein recognizing three operator sequences O1 centered at +11, O2 within lacZ, and O3 at -82 upstream. Primary O1 overlaps transcription start, so LacI binding sterically blocks RNA polymerase binding or promoter escape, repressing initiation. Loop formation via simultaneous O1-O2 or O1-O3 binding increases cooperativity hundred-fold enhancing repression tightness and stability. Inducer binding to core domain triggers allosteric change incompatible with operator groove interaction releasing DNA. CAP site upstream and promoter -10 elements serve as activator and polymerase docking sites, not LacI targets in regulation.