Iron acquisition and its control in Pseudomonas aeruginosa: many roads lead to Rome

Iron plays an important role in the pathogenesis and rhizosphere competence of the fluorescent group of pseudomonads and it is, thus, fitting that the characteristic fluorescence of these organisms is attributable to an iron-chelating molecule, pyoverdine. Pseudomonas aeruginosa is likely the best-studied member of this group, and while it synthesizes two siderophores, pyochelin and pyoverdine, it is also able to use a number of heterologous siderophores of fungal and bacterial origin and its genome is rich with homologues of iron-siderophore receptor genes, reflecting the enormous flexibility of the organism vis-a-vis iron carriers that it can use in nature. The ability to utilize a variety of heterologous siderophores is shared by other fluorescent pseudomonads and likely reflects both the importance of this vital nutrient for growth and survival and the need to compete with other microorganisms in the aquatic and terrestrial environments that they inhabit. Expression of the various receptors is, however, regulated, with receptor production responding positively to available siderophores only, and selection from multiple available siderophores based on their successful chelation of iron and subsequent transport. Thus, the superior siderophore in a given environment will upregulate the cognate receptor at the expense of other receptors. Such siderophore-dependent regulation of receptor gene expression is common in bacteria, particularly the fluorescent pseudomonads, and typically requires a signal transduction cascade that involves the receptor itself, whose binding to the siderophore initiates the cascade, as well as a regulatory protein pair that includes an environmentally-responsive so-called extracytoplasmic function (ECF) sigma factor, which activates receptor gene expression, and an anti-sigma factor that controls sigma factor activity. Despite the plethora of ferric siderophore receptors in P. aeruginosa, its genome sequence reveals a striking lack of obvious periplasmic and cytoplasmic membrane transport components capable of accommodating these molecules. Unlike e.g. Escherichia coli, then, where ferric siderophore permeases providing transport to the cytoplasm are clearly in evidence, iron-siderophore complexes in P. aeruginosa may be dissociated in the periplasm, with a common iron carrier then responsible for iron uptake into the cell interior.