Second, the lac operon (meta) is in a repressed state in the absence of lactose. In this case, the Lac repressor expressed by the I sequence under the PI priming sequence binds to the O sequence, preventing the RNA polymerase from binding to the P sequence and inhibiting the transcription initiation. When there is lactose, lac operon (yuan) can be induced. In this operon (meta) system, the real inducer is not lactose itself. Lactose enters cells and catalyzed by β-galactosidase to convert to isolactose. The latter, as an inducer, binds to the repressor molecule and changes the conformation of the protein, resulting in the dissociation of the repressor protein from the O-sequence and transcription. Isopropylthiogalactoside (IPTG) is the same as lactose, is a highly effective inducer, which is not metabolized by bacteria and very stable, it is widely used in laboratories.

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