The operon is a group of genes coding for proteins with related functions and they are arranged in units. The operon is made up of the promoter, structural genes and the operator. Transcription depends on the regulator, it may be located anywhere on the bacterial chromosome. The regulator codes for the repressor, which binds to the operator. When the repressor is bound to the operator, it blocks the promoter. Therefore the RNA polymerase either can't bind to the DNA molecule or it can't begin its movement along the molecule. No transcription can occur. The repressor depends on the effector, which allows it to bind to the molecule. It can either activate or inactivate the repressor.
The lac operon in bacterial cells gave the first clue that such a gene occurred in cells. When lactose is present, the first step in its metabolism is to produce a closely related sugar which binds to and inactivates the repressor, removing it from the operator. RNA polymerase can begin its movement along the DNA molecule, and transcribe the operon into mRNA. The lac operon is inducible.
In the case of the trp operon, the presence of the amino acid activates the repressor, and blocks the synthesis of the unneeded enzymes. It is repressible. Both the lac and trp operons are examples of allosteric effectors. They exert their effects by causing a change in the configuration of the repressor molecule.