In the absence of lactose, the repressor protein encoded by I gene binds to the manipulation sequence O, and the lactose operon is in an inhibitory state, so it is impossible to synthesize three enzymes that decompose lactose.

In the presence of lactose, lactose, as an inducer, induces protein allosteric disorder, which can not be combined with the manipulation sequence, and induces the lactose operon to open up and synthesize three enzymes that decompose lactose. Therefore, the regulatory mechanism of lactose operon is to induce negative regulation.

Structural genes related to bacterial functions are usually linked together to form gene clusters. They encode different enzymes in the same metabolic pathway. Gene clusters are regulated by identity, openness and closeness. In other words, they formed a supervised unit.

Other related functional genes are also included in the regulatory unit, such as genes encoding enzymes. Although their products are not directly involved in catalytic metabolism, they can transport small molecular substrates into cells.

Extended data:

Development of lactose operon;

The existence of special substrates leads to the synthesis of enzymes, which is called induction. This type of regulation exists widely in bacteria and also in lower eukaryotes. The lactose operon of Escherichia coli provides a typical example of this regulatory mechanism.

When Escherichia coli grows without β-galactosidase, it does not need β-galactosidase, so the intracellular content is very low, about no more than 5 molecules per cell. When the substrate is added, the enzyme can be synthesized very quickly in bacteria, which can be produced within 2-3 minutes and grow rapidly to 5000 molecules per cell. For example, the concentration of enzyme will reach 5- 10% of the total protein of cells.

If the substrate is removed from the culture medium, the synthesis of the enzyme will soon stop and return to its original state.

If the original medium does not contain lactose and glucose, the cells only synthesize β -galactosidase and osmotic enzyme at a very low level. When Lac was added, Lac+ cells of Ecoli quickly synthesized the above two enzymes in large quantities.

In addition, the use of 32P-labeled mRNA as a hybridization experiment (hybridization with DNA obtained in λlac and molecules of 32P-mRNA produced at different times after adding lactose) showed that the added lactose could stimulate the synthesis of lac mRNA. Lac mRNA is very unstable, with a half-life of only 3 minutes, which can be quickly recovered by induction.

When the transcription of inducer removal was stopped immediately, all lac mRNA was degraded in a short time, and the intracellular content returned to the basic level.

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