*The lac operon consists of 3 structural genes, and a promoter, a terminator, regulator, and an operator. The three structural genes are: lacZ, lacY, and lacA.
**lacZ encodes β-galactosidase (LacZ), an intracellular enzyme  that cleaves the disaccharide lactose into glucose and galactose.
** lacY encodes Beta-galactoside permease (LacY),  a transmembrane symporter that pumps β-galactosides including lactose into the cell using a proton gradient in the same direction. Permease increases the permeability of the cell to β-galactosides.
** lacA encodes β-galactoside transacetylase (LacA), an enzyme that transfers an acetyl group from acetyl-CoA to thiogalactoside.
Only lacZ and lacY appear to be necessary for lactose catabolic pathway.