RNA synthesis from the structural genes is controlled by normal regulatory sequences. The normal lac repressor is transcribed from the lac I gene on the bacterial chromosome. The repressor protein is constitutively expressed, and accumulates at a level of approximately 10 molecules/cell. Repressor proteins will bind to the operators on both the bacterial chromosome and F' episome when lactose is absent, preventing synthesis of mRNA.
When the inducer, either lactose or allolactose is present, the inducer binds to the repressor causing an allosteric switch to a conformation that does not bind the operator DNA sequence. Transcription from the plac promoters on both the chromosome and F' episome is initiated.