Why must E coli retain b-galactosidase enzymes if they have been growing in a lactose free medium?

Question

Why must E coli retain b-galactosidase enzymes if they have been growing in a lactose free medium?

Answer 1

That is a good question. Many people associate Beta-galactosidase with lactose only but in reality this enzyme break other beta-galactosides  into sugar monomers, not only lactose.

Answer 2

In the absence of lactose the cell still has a few galactosidase molecules , which would seem to have little value. However, the entire lac operon also controls the permease gene. Without the permease no lactose can enter the cell which would mean that the operon would never get turned on, even when lactose is present. Since the entire operon turns on and of as a unit, in order to produce a small number of permease molecules the operon also creates a small number of galactosidase molecules at the same time.