An operon is a regulatory unit consisting of structural genes under the control of a promoter. An operon encodes an mRNA that contains the coding sequence for one or more structural genes. An operon contains several important DNA sequences, including a promoter, structural genes, an operator, and a terminator. During the cycle of lac operon induction and repression, when lactose is present, allolactose is released from the operator, allowing it to bind to the operator site.