Can you think for how long the lac operon would be expressed in the presence of lactose?
The lac operon would be expressed as long as lactose is present in the growth medium. Lactose acts as both the inducer and the substrate for this operon.
When lactose is present, it interacts with and inactivates the repressor protein synthesized by the i gene. This allows RNA polymerase to transcribe the structural genes (z, y, and a). The enzyme beta-galactosidase (product of the z gene) then hydrolyzes lactose into glucose and galactose.
As lactose gets metabolized and depleted, there is no inducer left to inactivate the repressor. The repressor then rebinds to the operator region, blocking RNA polymerase and switching off the operon.
Explanation
The textbook establishes that lactose serves dual roles in the lac operon: it is both the inducer (which inactivates the repressor) AND the substrate (which gets metabolized by beta-galactosidase). This creates a self-regulating feedback loop. When lactose is present, it induces the operon, producing enzymes that break it down. As lactose is consumed and depleted, the induction stops, the repressor rebinds to the operator, and the operon switches off automatically. This is an elegant example of substrate-level regulation of gene expression.
Solution Steps
Step 1: Identify that lactose is both the inducer and substrate for the lac operon
Step 2: Understand that beta-galactosidase hydrolyzes/metabolizes lactose into glucose and galactose
Step 3: Conclude that operon expression continues only until lactose is depleted from the medium