Fig. 2.

Regulatory circuits controlling expression of the alcaligin, enterobactin, and heme utilization systems in Bordetella species. Fur and iron control the expression of three parallel regulatory circuits that respond to the cognate iron source via distinct substrate-inducible positive regulators (AlcR, BfeR, and HurI). The circuits are activated independently, and are not subject to interference or signal antagonism. All three circuits converge to supply the requisite nutrient iron. When the bacterium’s iron demands are satisfied, iron activates the DNA-binding activity of the Fur repressor and iron transport system expression is globally repressed