inducer

Meaning: An inducer is a small molecule that switches on transcription in an inducible system. In the classic bacterial case, a repressor protein sits on the operator and blocks RNA polymerase. The inducer binds the repressor, changes its shape (an allosteric change), and the repressor lets go of the operator, so transcription proceeds. In the lac operon the natural inducer is allolactose, an isomer of lactose. IPTG is a lab inducer that the cell cannot metabolize, so its level stays steady.

On the MCAT: Passages describe mutants and growth conditions and ask whether β-galactosidase is made. A lacI mutant whose repressor cannot bind the inducer stays on the operator, so there is no expression even with lactose. An operator mutant that the repressor cannot bind gives constitutive expression. Full lac expression also needs low glucose: high cAMP lets CAP-cAMP help RNA polymerase, so lactose plus glucose gives low expression.

Common confusion: Inducer versus corepressor. An inducer inactivates a repressor, as in inducible operons such as lac, which are normally off. A corepressor, such as tryptophan in the trp operon, binds the repressor to activate it and shuts the operon off. Also, cAMP is not the lac inducer; it acts through CAP.