The two French scientists, Jacob and Monod proposed the lac operon of E. coli. The lac operon (an inducible operon) contains a promoter, an operator, a regulator gene and three structural genes z,y, and a, coding for the enzyme β -galactosidase, β-galactoside permease, and β -galactoside transacetylase, respectively. β -galactoside permease "pumps" lactose into the cell, where β-galactosidase cleaves it into glucose and galactose.