Final answer:
An inducible operon is stimulated when a specific substrate like lactose is present, as seen in E. coli's lac operon which starts transcription when lactose converts to allolactose, acting as an inducer.
Step-by-step explanation:
An inducible operon is usually off but can be stimulated (induced) when a specific substrate, such as lactose, is present. For example, in the lac operon of E. coli, the presence of lactose induces the expression of the operon. When lactose is taken up by the cell, it is converted into allolactose, which then acts as an inducer by binding to the repressor and preventing it from blocking the operon. This allows RNA polymerase to access the operon's genes and initiate transcription. Additionally, the presence of an activator, the CAMP-CAP complex, is required for induction when glucose levels are low to initiate the transcription of the lac operon.