The lac operon is an operon required for the transport and metabolism of lactose in E.coli. When lactose is added to the growth medium, the lac genes are expressed because allolactose binds to the Lac repressor protein and keeps it from binding to the lac operator. Allolactose is an isomer of lactose. Small amounts of allolactose are formed when lactose enters into E. coli. Allolactose binds to the repressor protein and causes the conformational change. As a result of this, the repressor can no longer bind to the operator region and falls off. RNA polymerase can then bind to the promoter and transcribe the lac genes. After some time, when the level of lactose decreases as it is completely metabolized by enzymes, it causes synthesis of the repressor from the regulator gene. This repressor binds to the operator gene and prevents RNA polymerase from transcribing the operon and the transcription is stopped. This type of regulation is known as negative regulation.