The lac operon in *E. coli* is a genetic regulatory system that controls the utilization of lactose as an energy source. It consists of genes encoding enzymes necessary for lactose uptake and metabolism, such as β-galactosidase. When lactose is absent, the operon is repressed to conserve energy. In the presence of lactose, it induces the expression of these genes, enabling the bacteria to metabolize lactose efficiently. This precise regulation exemplifies gene expression control in prokaryotes.

Reference: Molecular Biology of the Cell, Bruce Alberts, 6th Edition.