Gene regulation of siderophore-mediated iron acquisition in Pseudomonas: not only the Fur repressor

Pseudomonads have several siderophore-mediated iron-acquisition systems. These can be classified onto two groups: (i) the biosynthesis of a siderophore (iron-transport agent) followed by its uptake; and (ii) uptake of heterologous ferric-siderophores in which the siderophore is produced by other microbial species. The regulation of these mechanisms employs both positive and negative elements ensuring expression of the relevant genes only when they are absolutely required. Siderophore biosynthesis in induced in response to iron limitation. In contrast, activation of the heterologous transport systems is not only regulated by iron availability but also requires the presence of their cognate ferric-siderophores. The investigation of these regulation systems in three different Pseudomonas species gave similar results consisting of regulatory elements new to the field of iron regulation. These elements are superimposed upon the regulation by the Fur repressor, which in other bacteria directly regulate the expression of the iron-assimilation genes in response to iron availability.