FIG. 4.
Differential regulation of ETEC enterotoxins by CRP. (A) In the presence of its ligand cAMP, CRP activates the expression of heat-stable toxin STa and represses the expression of heat-labile toxin LT-I. High concentrations of glucose suppress the synthesis of cAMP, rendering CRP inactive. (B) As a result, the expression of the enterotoxins may change relative to one another as the pathogen moves through the small intestine because glucose concentrations are typically higher in the duodenum than in the ileum. CRP without its cAMP cofactor is represented as CRPapo.