Answer and Explanation:
Bacteria like E.coli have devised mechanisms of survival even when conditions are harsh. This is through the production of such enzymes as B-galactosidase that enable them break down substrates like lactose in their metabolism.
For E.coli, mechanism that controls how enzyme activity is turned on or off target the genes that code for the enzymes. This system is termed as lac operon, and the enzymes that are coded for by genes which are sometimes active or inactive and depending on substrate availability are referred to as inducible enzymes.
So how does the lac operon work?
Lac operon works to regulate lactose metabolism in E.coli. An operon refers to a collection of one or more structural genes that direct synthesis of a particular enzyme.
So when lactose is present in a growth media, it acts as an Inducer hence it binds to lac oppressor (normally blocks operator). This reppressor then cannot block the operator, and RNA polymerase enzyme can proceed and bind the promoter hence transcription of the Z, Y and A genes that form the mRNA.
The mRNA in association with ribosomes initiate the synthesis of the following enzymes for lactose metabolism;
Permease, Y gene - that normally directs the entry of lactose into the cells
B-galactosidase, Z gene - that aim to break down lactose to simple units
Transacetylase, A gene - for complete lactose breakdown.
So after these events in E.coli cell, the lactose that was once available has been broken down and none is left to bind to repressor. What happens thereafter is the binding of this active repressor to the operator and thus system is shut down or more correctly, turned off.
