GENE-REGULATION OF SIDEROPHORE-MEDIATED IRON ACQUISITION IN PSEUDOMONAS - NOT ONLY THE FUR REPRESSOR

Pseudomonads have several siderophore-mediated iron-acquisition system s. These can be classified into two groups: (i) the biosynthesis of a siderophore (iron-transport agent) followed by its uptake; and (ii) up take of heterologous ferric-siderophores in which the siderophore is p roduced by other microbial species. The regulation of these mechanisms employs both positive and negative elements ensuring expression of th e relevant genes only when they are absolutely required. Siderophore b iosynthesis is induced in response to iron limitation. In contrast, ac tivation of the heterologous transport systems is not only regulated b y iron availability but also requires the presence of their cognate fe rric-siderophores. The investigation of these regulation systems in th ree different Pseudomonas species gave similar results consisting of r egulatory elements new to the field of iron regulation. These elements are superimposed upon the regulation by the Fur repressor, which in o ther bacteria directly regulate the expression of the iron-assimilatio n genes in response to iron availability.