Topic overview

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The lac operon’s structural genes—lacZ, lacY, and lacA—are transcribed together as one polycistronic mRNA. When lactose is absent, the lacI-encoded repressor binds the operator and blocks transcription. Allolactose changes the repressor’s conformation, allowing it to leave the operator. A second control system responds to glucose: low glucose raises cAMP, enabling cAMP–CAP to promote RNA polymerase binding. Strong expression therefore depends on both relief of repression and positive activation. Mutations reveal how different parts of the system contribute: lacI⁻ and lacOᶜ can cause constitutive expression, lacIˢ keeps the operon off, and promoter or activation defects reduce transcription. Together, these mechanisms illustrate the operon’s AND-gate logic.