How lactose regulates the lac repressor

A key feature of the operon model is the idea that genes with metabolically related functions are clustered together so their transcription can be regulated as a single unit.

- TAPAN KUMAR MAITRA

But how is this regulation accomplished? Jacob and Monod addressed this question by studying the ability of inducers, such as lactose, to turn on the production of the enzymes involved in lactose metabolism. They found that for induction to occur, an additional gene must be present—a regulatory gene that they named lacl (for inducibility). Whereas normal bacteria will produce B-galactosidase, galactoside permease, and transacetylase only when an inducer is present, deletion of the lad gene yields cells that always produce these proteins, even when an inducer is absent. Jacob and Monod therefore concluded that the lad gene codes for a product that normally inhibits, and thereby regulates, expression of the lacZ, lacy, and lacA genes. A regulatory gene product that inhibits the expression of other genes is called a repressor protein.