The lac operon is a classic example of gene regulation in prokaryotes. It consists of three structural genes: lacZ, lacY, and lacA, which encode enzymes needed for lactose metabolism. The system includes regulatory elements like the promoter, operator, and the lacI gene that produces a repressor protein.
The lac repressor provides negative control of the operon. Without lactose, the repressor protein binds tightly to the operator sequence, physically blocking RNA polymerase from transcribing the genes. When lactose is present, it is converted to allolactose, which binds to the repressor and causes it to release from the operator, allowing transcription to proceed.