In the presence of both sugars (glucose and lactose) will there be repression of the lac operon completely?

I know that more glucose means less cAMP --> less CAP --> less positive regulation, and thus less lac operon, but does that mean there will be repression of the lac operon?

  • $\begingroup$ There is never a complete absence of glucose in a living being; the issue is with the amount available and amount required difference. This difference determines the rate of expression of the Lac genes after the lac repressor has been inactivated due to the presence of lactose. $\endgroup$
    – Atajera
    Commented Jan 8, 2019 at 20:01

1 Answer 1


According to Molecular biology of the gene by James Watson, in presence of both glucose and lactose, the lac operon has a basal level of transcription. However, when only lactose is present, the operon has an activated level of transcription, due to the CAP mechanism.


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