In the medium where E. coli was growing, lactose was added, which induced the lac operon. Then, why does lac operon shut down some time after addition of lactose in the medium?
When lactose is added to the medium, it acts as an inducer and inactivates the repressor protein by interacting with it. This allows RNA polymerase access to the promoter and transcription of the lac operon proceeds, producing enzymes like beta-galactosidase.
The enzyme beta-galactosidase is responsible for the hydrolysis of lactose into its monomeric units, galactose and glucose. As lactose is continuously metabolised by the enzyme, the concentration of the inducer (lactose) in the medium decreases.
When lactose is fully depleted from the medium, the repressor is no longer inactivated. The repressor becomes free to bind to the operator region of the operon again, which prevents RNA polymerase from transcribing the operon. This results in the shutting down of the lac operon.
Explanation
The answer explains the self-regulating nature of the lac operon. The key concept from the textbook is that lactose acts both as a substrate and an inducer. When beta-galactosidase (coded by the z gene) hydrolyses lactose into galactose and glucose, the inducer molecule itself is consumed. Without the inducer present, the repressor protein (synthesised constitutively from the i gene) regains its ability to bind to the operator region and block transcription. This is an example of negative regulation where the operon is expressed only as long as the substrate/inducer is available.
Solution Steps
Step 1: Identify that lactose acts as an inducer that inactivates the repressor protein
Step 2: Explain that beta-galactosidase enzyme hydrolyses lactose into galactose and glucose
Step 3: Conclude that depletion of lactose (inducer) allows repressor to bind operator again, shutting down the operon