Which statement correctly describes how lactose regulation of lac operon works in the presence of lactose?

Which statement correctly describes how lactose regulation of lac operon works in the presence of lactose?

• A. The lac repressor binds the operator to block transcription regardless of lactose presence.
• B. Lactose directly activates RNA polymerase independent of repressor.
• C. Allolactose (inducer) binds the lac repressor, releasing it and allowing transcription.
• D. cAMP-CRP complex inhibits RNA polymerase binding under lactose presence.

Answer: (C)

When lactose is available, it is converted to allolactose, which acts as the inducer by binding the lac repressor. This binding changes the repressor so it can no longer attach to the operator, removing the block on transcription and allowing RNA polymerase to transcribe the lac operon genes. So transcription proceeds because the repressor is derepressed, not because lactose directly activates RNA polymerase. The repressor only blocks transcription in the absence of lactose. The cAMP-CRP complex can enhance RNA polymerase binding under low glucose, but it does not inhibit transcription in the presence of lactose.

When lactose is available, it is converted to allolactose, which acts as the inducer by binding the lac repressor. This binding changes the repressor so it can no longer attach to the operator, removing the block on transcription and allowing RNA polymerase to transcribe the lac operon genes. So transcription proceeds because the repressor is derepressed, not because lactose directly activates RNA polymerase. The repressor only blocks transcription in the absence of lactose. The cAMP-CRP complex can enhance RNA polymerase binding under low glucose, but it does not inhibit transcription in the presence of lactose.