| Lac Repressor |
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FUNCTION The ''lac'' repressor (LacI) operates by binding to the promoter 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 to the ''lac'' promoter. )]] STRUCTURE The lac repressor protein has three distinct regions:
The lac repressor occurs as a Tetramer (four identical subunits bound together). This can be viewed as two Dimer s, with each dimer being able to bind to a single ''lac'' operator. The two subunits each bind to a slightly separated (major groove) region of the operator. The promoter is slightly covered by the lac repressor so RNAP cannot bind to and transcribe the operon. The DNA binding region consists of a Helix-turn-helix Structural Motif . DISCOVERY The ''lac'' repressor was first Isolated by Walter Gilbert and Benno Müller-Hill in 1966 . They were able to show, '' In Vitro '', that the protein bound to DNA containing the ''lac'' operon, and released the DNA when IPTG was added. (IPTG is an allolactose Analog .) They were also able to isolate the portion of DNA bound by the protein by using the Enzyme Deoxyribonuclease , which breaks down DNA. After treatment of the repressor-DNA complex, some DNA remained, suggesting that it had been masked by the repressor. This was later confirmed. These experiments were important, as they confirmed the mechanism of the ''lac'' operon, earlier proposed by Jacques Monod and Francois Jacob . SEE ALSO |
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