The lac operon (lactose operon) is an operon required for the transport and metabolism of lactose in some prokaryotes. When lactose is present, this operon is functional and produces the enzymes and proteins required for lactose metabolism; when lactose is absent, the operon is prevented from functioning.
When lactose is present,
binds to the promoter upstream of the genes necessary for lactose metabolism and begins transcription. When lactose is absent,
binds to the promoter, inhibiting the process of transcription of the genes.