What type of operon is usually off and requires induction to be activated?

Negative inducible operons are typically in an "off" state and require an inducer to become activated. This type of operon is regulated by a repressor protein that binds to the operator region, preventing transcription when there is no inducer present. When an inducer molecule comes into contact with the repressor, it causes a conformational change that reduces the repressor's affinity for the operator. As a result, the repressor is released, allowing RNA polymerase to access the promoter and initiate transcription of the structural genes of the operon.

In the context of prokaryotic genes, this is particularly relevant for operons that are involved in the metabolism of specific substrates, where the presence of the substrate (the inducer) signals the cell that the necessary enzymes for its breakdown should be produced. For instance, the classic example of a negative inducible operon is the lac operon, which allows for the metabolism of lactose when it is present.

The other types of operons each have different regulatory mechanisms: positive inducible operons require an activator to enable transcription, negative repressible operons are typically "on" and can be turned off by a repressor when a corepressor is present, and positive repressible oper