* Three enzymes are needed for lactose metabolism in E.coli namely β- galactosidase, -galactoside permease and thio galactoside transacetylase.
* The three structural genes Z,Y and X are located adjacent to one another in a cluster (cluster genes) in the DNA.
* The promoter P is situated close to Z.
* The operator O is adjacent to P.
* The regulator R is not immediately close to O but located near it.
* In the inductive strain of E.coli, the regulator gene (R) produces a repressor protein (RP).
* Lactose functions as an inducer combines with RP and inactivates it.
* Since the repressor is inactivated, operon becomes activated, the switch gets to ‘on‘ mode.
* The operator which is free now initiates RNA polymerase enzyme to bind to the promoter gene.