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Lac operon --> lactose operon

group of adjacent and coordinately controlled genes concerned with the metabolism of lactose in E. Coli. The lac operon was the first example of a group of genes under the control of an operator region to which a lactose repressor binds. When the bacteria are transferred to lactose containing medium, allolactose (which forms by transglycosylation when lactose is present in the cell) binds to the repressor, inhibits the binding of the repressor to the operator and allows transcription of mrna for enzymes involved in galactose metabolism and transport across the membrane (_ galactosidase, galactoside permease and thiogalactoside transacetylase).