Explain the lac operon. How does it work when lactose is present and when lactose is not present. What about the presence of glucose?

The lac operon is an operon used to transport and metabolize lactose. In the presence of lactose, it functions as an inducer, it binds to the Lac I repressor protein and generate a conformational change that decreases its affinity for the operon region. Thus, the operon region is left free, the RNA polymerase can transcribe the structural genes.

Explanation:

An Operon is a group of structural genes whose expression is regulated by the same control elements and genes. The lac operon is an operon used to transport and metabolize lactose in Escherichia coli bacteria. Lactose is a disaccharide formed by the union of a glucose molecule and a galactose molecule (monomers). Upon breaking the glycosidic bond of lactose, both monomers are released and become available for use for energy. Remember that glucose is the main source of energy, and bacteria will always prefer to metabolize glucose rather than any other sugar, which they will only metabolize as an alternative energy source if glucose is not available.

The operon consists of several structural genes:

So, the lac operon is under a type of negative regulation, where genes can always be transcribed, except when the Lac I repressor protein is bound to the operator region. The promoter of the lac I gene is constitutive, so the Lac I protein is permanently expressed and remains bound to the operator, preventing the transcription of structural genes. That is, preventing the action of the lac operon in the absence of lactose. However, in the presence of lactose, it functions as an inducer of the operon. It is able to bind to the Lac I repressor protein and generate a conformational change that decreases its affinity for the operon region. Thus, the operon region is left free, the RNA polymerase can freely transcribe the structural genes, the enzyme β-galactosidase is synthesized which can degrade lactose to glucose plus galactose (and thus metabolize the lactose in the medium).

It should be clarified that in reality, the true inducing molecule of the lac operon is allolactose, an isomer of lactose obtained by a transglycosylation occasionally carried out by β-galactosidase. But usually to simplify terms, lactose is referred to as the inducer.

So, the lac operon is always inactive unless there is lactose in the medium. As mentioned, E. coli prefers glucose, so the lac operon will not be activated in the presence of lactose if glucose is present in the medium. If both glucose and lactose are present in the medium, the repressor is inactivated, so the operon is transcribed but at a basal (low) level due to the presence of glucose.

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