Explain the regulation of the lac operon in E. coli; which elements control its expression?

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Multiple Choice

Explain the regulation of the lac operon in E. coli; which elements control its expression?

Transcriptional control of the lac operon happens mainly at initiation through two regulators working in concert: a repressor and an activator, with an inducer tipping the balance. The LacI repressor binds to the operator and blocks RNA polymerase from starting transcription. When lactose is present, a derivative called allolactose binds LacI and changes its shape so it releases from the operator, relieving repression. At the same time, an activator complex formed by CAP and cAMP binds near the promoter to boost RNA polymerase binding and encourage transcription; this CAP-cAMP complex forms when glucose is scarce and cAMP levels are high, reinforcing expression. So you get robust lac operon expression only when lactose is available (removing repression via allolactose) and glucose is low (enabling CAP-cAMP activation). If lactose is absent, repression remains and transcription stays off regardless of CAP. If glucose is high, CAP-cAMP is not formed even with lactose, so transcription remains weak. This combination of LacI, allolactose, and CAP-cAMP is what best explains the regulation of the lac operon.

Transcriptional control of the lac operon happens mainly at initiation through two regulators working in concert: a repressor and an activator, with an inducer tipping the balance. The LacI repressor binds to the operator and blocks RNA polymerase from starting transcription. When lactose is present, a derivative called allolactose binds LacI and changes its shape so it releases from the operator, relieving repression. At the same time, an activator complex formed by CAP and cAMP binds near the promoter to boost RNA polymerase binding and encourage transcription; this CAP-cAMP complex forms when glucose is scarce and cAMP levels are high, reinforcing expression. So you get robust lac operon expression only when lactose is available (removing repression via allolactose) and glucose is low (enabling CAP-cAMP activation). If lactose is absent, repression remains and transcription stays off regardless of CAP. If glucose is high, CAP-cAMP is not formed even with lactose, so transcription remains weak. This combination of LacI, allolactose, and CAP-cAMP is what best explains the regulation of the lac operon.

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