In the absence of lactose, what happens to lac operon transcription?

• A. Repressor binds the operator to block transcription.
• B. The repressor is inactivated by allolactose, allowing transcription.
• C. cAMP-CRP complex binds promoter to increase transcription.
• D. Lactose acts as a corepressor to enhance transcription.

Answer: (A)

Without lactose, the lac operon stays off because the LacI repressor remains bound to the operator, blocking RNA polymerase from initiating transcription. Lactose would be converted to allolactose, which binds LacI and prevents it from attaching to the operator, allowing transcription to proceed. The cAMP-CRP system can enhance transcription when glucose is low, but it cannot overcome repression when the repressor is bound. Lactose does not act as a corepressor; it functions as the inducer that relieves repression.