Molecular switch in E.coli

A cell contains thousands of genes, but proteins encoded by all of them are not required by the cell at all times .How the cell's need for a new class of protein conveyed to the corresponding genes?or, for that matter, how are certain other genes told that their protein products are no longer needed?In other words how are genes switched on or off ? 

The answer to all these question is same with the help of molecular switches. A molecular switch usually consists of a single molecule which can shift controllable between two stable states. The trigger used to switch between the states can be an electrical current, a change in temperature or chemical environment, or even light.

It is known that some of enzymes required for breakdown of sugars appear in bacterial cells only when a particular sugar is provided to the bacteria as it's food. If the sugar is removed, the enzyme is no longer synthesized.

I' ll be taking example of molecular switch in E.coli.

If Ecoli cells are given lactose in their growth medium, they begin to synthesis the enzyme β-galactosidase (also called beta-gal or β-gal, is a hydrolase enzyme that catalyzes the hydrolysis of β-galactosides into monosaccharides) which breaks dowm lactose into glucose and galactose..However if another sugar is provide, in place of lactose these cells will not make  β-galactosidase, but instead make enzyme for synthesis of that sugar.

An Ecoli cell growing on lactose contains several thousand molecules of  β-galactosidase . Lactose induces a large increase in the amount of β-galactosidase and other 2 enzymes galactoside permease and thiogalactoside. all three enzymes stop being made when lactose is removed. here lactose act as inducer to initiate new synthesis of three enzyme

ThankYou 
with love
-Dixy