Promoter Gene - Promoter gene is present between the regulator and operator gene. RNA polymerase binds to promoter gene and initiates the transcription of the structural genes. RNA polymerase can initiate the transcription if the operator gene is free, i.e, repressor is not bound to the operator.

A binding site of another protein called catabolite activator protein (CAP), functions such that the lac operon is not transcribed in the presence of glucose at concentrations sufficient to support optimal growth.