inducible

Meaning: An inducible system is transcribed at a low basal level until an inducer appears. In an inducible operon, the repressor is typically bound to the operator and blocks RNA polymerase. The inducer binds the repressor, changes its shape, and releases it from the DNA, so transcription rises. The cell makes the enzymes only when their substrate is present, which fits catabolic pathways such as lactose breakdown. Contrast two other types: constitutive genes are expressed at a roughly steady rate regardless of conditions, and repressible genes (the trp operon) are typically on until the product acts as a corepressor and shuts them off.

On the MCAT: Passages usually describe lac operon mutants and ask you to predict β-galactosidase levels with and without lactose. A lacI mutant whose repressor cannot bind the operator gives constitutive expression. A repressor that cannot bind the inducer stays on the operator, so the genes stay off even with lactose. Graphs may add glucose: with lactose present, expression is high only when glucose is low, because cAMP and CAP then activate transcription. IPTG is a nonmetabolizable inducer analog used in experiments.

Common confusion: The inducer binds the repressor protein, not the operator, and it does not recruit polymerase directly. Inducible and repressible are not the same: in an inducible operon the repressor is active on its own, while in a repressible operon it needs a corepressor.