Enzymes responsible for the catabolism of many other organic molecules, including galactose, arabinose, sorbitol, and glycerol, are synthesized by inducible operons. Each of these operons cannot be induced if glucose is present. These are called catabolite-sensitive operons. A network of operons that is under the control of a single regulatory protein such as cAMP · CRP is called a modulon.
    A simple genetic experiment shows that cAMP · CRP participates in the regulation of many operons. A single spontaneous mutation in a gene for the catabolism of a sugar such as lactose, galactose, arabinose, or maltose arises with a frequency of roughly 10^-6. A double mutation, lac^-mal^-, would arise at a frequency of 10^-12, which for all practical purposes cannot be measured. However double mutants that are phenotypically Lac^- Mal^- or Gal^- Ara^- do arise at a measurable frequency. These apparent double mutants are not the result of mutations in the two sugar operons but always turn out to be crp^- or cya^-. Furthermore, if a Lac^- Mal^- mutant appears as a result of a single mutation, the protein products of the other catabolite-sensitive operons are also not synthesized. Biochemical experiments with a few of these catabolite-sensitive operons indicate that binding of cAMP · CRP occurs in the promoter region in each of these systems.