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The repressor protein binds to the operator region in the lac operon to block transcription in the absence of lactose.
Ref: NCERT Class 12 Biology Chapter 5: Molecular Basis of Inheritance Regulation of Gene Expression - Lac Operon
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
Ref: NCBI - Lac repressor binds operator sequence to block RNA polymerase; Lodish Fig 8-11 lac repression mechanism
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