If the function of a gene is to produce proteins, how do prokaryotic cells control when to produce and when not to?

E.Coli is an example of a prokaryote , it's a bacteria that lives in the large intestine of mammals, and it produces an enzyme called B-galactosidase (breaks down lactose into glucose and galactose). Because there won't always be milk in your large intestine, E.Coli developed a regulating expression of your gene called lac operon. Thus, prokaryotic cells use regulating expressions in gene to control the production of enzymes.