Function
The lac repressor (LacI) operates by binding to the major groove of the operator region of the lac operon. This blocks RNA polymerase from binding, and so prevents transcription of the mRNA coding for the Lac proteins. When lactose is present, allolactose binds to the lac repressor, causing an allosteric change in its shape. In its changed state, the lac repressor is unable to bind tightly to its cognate operator. This effect is referred to as induction, because it induces, rather than represses, expression of the metabolic genes. In vitro, Isopropyl β-D-1-thiogalactopyranoside (IPTG) is a commonly-used allolactose mimic which can be used to induce transcription of genes being regulated by lac repressor.
Read more about this topic: Lac Repressor
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