Iron is a common co-factor for redox-dependent enzymes and an essential element for almost all living organisms. In nature iron is abundant; however, in aerobic environments and under general physiological conditions, iron typically exists in the insoluble ferric (Fe3+) form, thus rendering acquisition by organisms difficult. Under conditions of iron limitation, many bacteria produce and secrete low molecular weight ferric-specific ligands known as siderophores (for review, see ). The ferric-siderophores deliver iron to the cell via specific receptor and transport systems (for review, see ).
Collectively, Pseudomonas spp. produce a wide variety of siderophores , the most complex and common of which are pyoverdines (PVDs) . PVDs contain a peptide moiety, usually between 6–12 amino acids in length, and a dihydroxyquinoline chromophore moiety , which gives PVD its characteristic yellow-green fluorescent appearance. PVD-mediated iron uptake processes have been extensively characterized in Pseudomonas aeruginosa PAO1, where key regulatory and structural proteins have been identified, and regulatory mechanisms have been elucidated [5–9].
In P. aeruginosa PAO1, the primary level of PVD regulation involves the ferric uptake regulator (Fur), which, upon interaction with ferrous iron (Fe2+), binds to a specific DNA sequence (the Fur-box) in the promoter region of certain iron-regulated genes and blocks transcription. Under iron-deplete conditions (no Fe2+ available to interact with Fur) Fur-dependent repression is relieved [6, 10]. PAO1 PVD genes that possess Fur boxes, which have been demonstrated to bind Fur in vitro [5, 11], are pvdS and fpvI, which encode extracytoplasmic family (ECF) sigma factors, and fpvR, an inner membrane-spanning anti-sigma factor. The alternative sigma factor, PvdS, when associated with core RNA polymerase, binds iron-starvation (IS) box motifs  and directs the expression of a suite of genes involved in the synthesis of PVD [7, 13]. The activity of PvdS is regulated by a transmembrane signaling system that comprises of FpvR and FpvA, an outer membrane PVD receptor, and is primarily mediated by PVD . More recently, a second ECF sigma factor (FpvI) was found to direct transcription of fpvA [5, 14]. Like PvdS, FpvI is also under the direct regulation of FpvR and forms a second divergent branch of the signaling pathway.
Many plant-associated pseudomonads are known to produce PVDs, including the plant pathogen, P. syringae and the saprophytes, P. putida and P. fluorescens. The importance of siderophores for rhizosphere colonization by P. putida has been previously reported [15, 16]. However, little is known about the molecular mechanisms of PVD production and regulation in these organisms.
Here we report a genetic characterization of genes for PVD production in a plant growth-promoting bacterium, Pseudomonas fluorescens SBW25. SBW25 was originally isolated from field-grown sugar beet  and its biocontrol activity against the soil-borne pathogen, Pythium ultimum, is related to its considerable plant colonization ability . The present investigation was prompted by initial findings that three genes implicated in iron-acquisition processes were up-regulated on plant surfaces, as revealed by in vivo expression technology (IVET) analysis [19, 20]. Two genes had significant homology to putative siderophore receptor genes, and one gene appeared to be homologous to PAO1 pvdL, which encodes a non-ribosomal peptide synthetase (NRPS) involved in PVD biosynthesis . These findings suggested a significant role for iron uptake during seedling colonization.
Our work began with the un-annotated whole genome sequence of SBW25 . Based initially on an analysis of this sequence, 31 genes were identified with predicted roles in PVD biosynthesis, transport and regulation, for which the regulation of a subset of these was investigated by genetic analysis using chromosomally-integrated 'lacZ fusions and gene expression constructs. The results indicate a not previously realized mechanism of fpvR regulation. Moreover, we determine the chemical structure of SBW25 PVD and examine the biological role of PVD via a siderophore-deficient mutant. The ability of SBW25 to utilize a panel of structurally distinct exogenous PVDs was also assessed.