the example of lactose (Lac-Operon)
Substrate induction is based on the Operon model of Jacob and Monod.
If lactose is present in the cell in the prokaryote E. coli, it binds to the repressor, which is thereby inactivated. This allows the RNA polymerase to run off and synthesizes the three enzymes lacZ, lacY, lacA to break down the lactose. However, if there is no lactose in the cell, the repressor binds to the operator and prevents the transcription from proceeding to the synthesis of lactose-degrading enzymes.
From the point of view of the cell, the process of gene regulation ensures the economical use of resources, because only lactose-degrading enzymes are formed when lactose is present in the cell.
No lactose in the cell: Repressor binds to the operator and prevents the transcription of lactose-degrading enzymes (repressor active)
Lactose in the cell: Lactose molecules bind to the repressor and thereby change its structure (repressor inactive). The RNA polymerase can drain and synthesize the lactose-degrading enzymes.