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E. Coli's lactose operon (element) contains three structural genes, Z, Y and A, which encode galactosidase, transaminase and acetyltransferase, respectively. In addition, there is a manipulation sequence O, a promoter sequence P and a regulatory gene I (Figure 15-4). The I gene encodes a repressor protein, which binds to the O sequence and causes the operon to be repressed and shut down. There is also a cap binding site upstream of the promoter P. The regulatory region of lac operon is composed of P sequence, O sequence and cap binding site. The coding genes of the three enzymes are regulated by the same regulatory region to achieve the coordinated expression of gene products.

In the absence of lactose, lac operon is in a repressive state. At this time, the lac repressor expressed by I sequence under the control of PI promoter binds to O sequence, hinders the binding of RNA polymerase to P sequence, and inhibits transcription initiation. When lactose is present, lac operon can be induced. In this operon system, the real inducer is not lactose itself. Lactose enters the cell and is converted into galactose by b-galactosidase. The latter, as an inducer, binds to the repressor protein and changes its conformation, leading to the dissociation and transcription of the repressor protein and O sequence. Isopropylthiogalactoside (IPTG) is a strong inducer, which is not metabolized by bacteria and is very stable, so it is widely used in the laboratory.