text: In bacteria, the level of cAMP varies depending on the medium used for growth.  In particular, cAMP is low when glucose is the carbon source.  This occurs through inhibition of the cAMP-producing enzyme, adenylate cyclase, as a side-effect of glucose transport into the cell.  The transcription factor cAMP receptor protein (CRP) also called CAP (catabolite gene activator protein) forms a complex with cAMP and thereby is activated to bind to DNA.  CRP-cAMP increases expression of a large number of genes, including some encoding enzymes that can supply energy independent of glucose.
  cAMP, for example, is involved in the positive regulation of the lac operon. In an environment with a low glucose concentration, cAMP accumulates and binds to the allosteric site on CRP (cAMP receptor protein), a transcription activator protein. The protein assumes its active shape and binds to a specific site upstream of the lac promoter, making it easier for RNA polymerase to bind to the adjacent promoter to start transcription of the lac operon, increasing the rate of lac operon transcription. With a high glucose concentration, the cAMP concentration decreases, and the CRP disengages from the lac operon.