1. Lac Repressor Protein: A protein called the lac repressor binds to a specific DNA sequence called the lac operator, located near the lac genes.
2. Operator Binding: When the repressor is bound to the operator, it physically blocks the RNA polymerase from accessing the promoter region, preventing the transcription of the lac genes.
3. Lactose Absence: In the absence of lactose, the lac repressor is bound to the operator, keeping the lac genes switched off. This is because the repressor has a high affinity for the operator in the absence of lactose.
4. Lactose Presence: When lactose is present in the environment, it binds to the lac repressor, causing a conformational change in the repressor protein.
5. Repressor Inactivation: This conformational change reduces the repressor's affinity for the operator, causing it to detach.
6. Transcription Initiation: With the repressor removed, RNA polymerase can now bind to the promoter and initiate the transcription of the lac genes, allowing the production of the enzymes needed to metabolize lactose.
In summary: The lac operon is regulated by a negative feedback loop, where the presence of lactose triggers the inactivation of the repressor, allowing the expression of the lac genes only when lactose is available. This efficient regulation ensures that E. coli only produces the enzymes needed to metabolize lactose when it's necessary, saving energy and resources.
