The lac operon is a gene system characterized in Escherichia coli for the regulation of the gene for utilization of lactose. If lactose is absent, the gene is turned off. If lactose is present, the gene is turned on. In the absence of lactose, a repressor protein (made by the lacI gene) binds to the operator region upstream of the lacZ, lacY and lacA genes and prevents the lactose utilization gene from being transcribed. Note that the operator region is just downstream of the promoter where RNA polymerase binds to start transcription. In the presence of lactose, a form of lactose binds to the repressor protein and it can’t bind to the operator, allowing transcription of the lacZYA region and production of an mRNA which is translated into proteins for lactose utilization, thus allowing the bacterium to utilize lactose.