The lac repressor is responsible for regulating the metabolism of lactose. In the absence of lactose, LacI forms a tetramer with identical subunits which appears as two dimers. Each dimer binds in the major groove of the DNA binding region which subsequently blocks the RNA polymerase from binding. In nature, allolactose will bind the repressor leading to transcription of the lac operon. Using IPTG as an inducer has the same effect as allolactose.

3D model

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