FIG. 19.19 The CRP–cAMP complex, a positive regulator of the lactose operon. (a) In the absence of large amounts of glucose, cAMP levels are high and the CRP–cAMP complex binds to a site near the promoter, where it activates transcription. (b) In the presence of large amounts of glucose, cAMP levels are low and the CRP–cAMP complex does not bind, so transcription is not induced to high levels, even in the presence of lactose.