With low levels of both glucose and lactose around, the low level of glucose leads to high levels of cAMP, helping RNA polymerase bind to the promoter of the lac operon. However, this would also lead to the repressor binding to the operator, blocking RNA polymerase from actually transcribing the operon. Thus, the cell would be poised, ready for high levels of transcription as soon as allolactose is available and leads to the unbinding of the repressor from the operator.
Work Step by Step
They key here is to put together the information in Fig.'s 18.3-18.5 to see the effects of both systems (CAP and repressor) on the lac operon. This shows how multiple inputs affect the expression of one operon.