The lac opcron found in E.coli is an inducible system responsible for the synthesis of enzymes involved in lactose (the milk sugar). It has an operator sequence of 26 base pairs and three structural genes.

The first structural gene (SG) is lac / of 3063 base pair and is responsible for the synthesis of the enzyme fi-galactosidasc. The operator is a part of lac z. The other two genes are lacy for sythesis of P-galactosidc permease and lac a for |3-galactosidc transacctylasc. [}-Galactosidc permease is a transmembrane protein that pumps galactose into the cell and a- galacto- sidase breaks lactose to galactose and glucose.

The function of galactoside transacetylase is not clear. When lactose is available to the bacterium the active repressor produced by the regulator
forms an inactive dimmer with lactose as lactose, the substrate of the enzyme galactosidase acts as an inducer.

This inactive dimer cannot bind to the operator and the three contiguous structural genes are transcribed into a polycistronic or polygenic mRXA. This polycistronic mRXA is translated into three proteins (enzymes). In the absence of lactose, the induccr, the product of regolator forms active inhibitor dimmer, that binds to the operator and prevents transcription.