Final answer:
The lac operon is termed a negative inducible system because a repressor inhibits gene transcription unless lactose is present, which induces transcription by inactivating the repressor. This system allows efficient metabolic adaptation to available nutrients, particularly under conditions of low glucose.
Step-by-step explanation:
The lac Operon: Negative Regulation and Inducibility
The lac operon is an example of a negative inducible genetic system because it is regulated by a repressor protein that binds to the operator to prevent transcription. When lactose is present, it is converted into allolactose, which acts as an inducer. The allolactose binds to the repressor, thereby preventing the repressor from binding to the operator and allowing transcription of the structural genes to proceed. Additionally, the operon is also subject to activation through the catabolite activator protein (CAP) when glucose levels are low.
The presence of lactose induces the transcription of genes within the operon. This is essential for metabolizing lactose because it would be wasteful for the cell to produce enzymes when lactose is not present. In the absence of glucose, cAMP levels increase, and the cAMP-CAP complex further activates transcription of the lac operon, enhancing the utilization of lactose when glucose is scarce.
In essence, the lac operon system allows the bacterial cell to adapt to changes in environmental glucose and lactose availability through a finely-tuned regulatory system, hence being described as 'negative' due to the repressor's role and 'inducible' because its transcription is upregulated in the presence of lactose.