The lac operon in the prokaryote E. coli consists of genes that produce enzymes to break down lactose. Its operon is an example of a prokaryotic silencer. The three functional genes in this operon are lacZ, lacY, and lacA. The repressor gene, lacI, will produce the repressor protein LacI which is under allosteric regulation. These genes are activated by the presence of lactose in the cell which acts as an effector molecule that binds to LacI.  When the repressor is bound to lactose, it will not bind to the operator, which allows RNA polymerase to bind to the promoter to initiate transcription of the operon. When the repressors allosteric site is not bound to lactose, its active site will bind to the operator to prevent RNA polymerase from transcribing the genes of the lac' operon.