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• lacZ coding for Β-galactosidase: hydrolyses lactose into glucose and galactose.
• lacY coding for permease: allows lactose to enter the cell.
• lacA coding for transactylase
• • The lac operon will only be turned on in the absence of glucose.
  • When glucose is absent cAMP accumulates.
  • cAMP combines with a molecule called catabolite activator protein (CAP). The complex then binds to CAP binding site next to the lac promoter.
  • The DNA bends exposing the promoter to RNA polymerase.
  • In the presence of lactose and absence of glucose the lac structural genes are transcribed leading to their expression.
  • When glucose is present there is little cAMP in the cell and CAP is inactive and the lac operon is turned off.