The Heterologous Siderophores Ferrioxamine B and Ferrichrome Activate Signaling Pathways inPseudomonas aeruginosa

Abstract

Pseudomonas aeruginosa secretes two siderophores, pyoverdine and pyochelin, under iron-limiting conditions. These siderophores are recognized at the cell surface by specific outer membrane receptors, also known as TonB-dependent receptors. In addition, this bacterium is also able to incorporate many heterologous siderophores of bacterial or fungal origin, which is reflected by the presence of 32 additional genes encoding putative TonB-dependent receptors. In this work, we have used a proteomic approach to identify the inducing conditions for P. aeruginosa TonB-dependent receptors. In total, 11 of these receptors could be discerned under various conditions. Two of them are only produced in the presence of the hydroxamate siderophores ferrioxamine B and ferrichrome. Regulation of their synthesis is affected by both iron and the presence of a cognate siderophore. Analysis of the P. aeruginosa genome showed that both receptor genes are located next to a regulatory locus encoding an extracytoplasmic function sigma factor and a transmembrane sensor. The involvement of this putative regulatory locus in the specific induction of the ferrioxamine B and ferrichrome receptors has been demonstrated. These results show that P. aeruginosa has evolved multiple specific regulatory systems to allow the regulation of TonB-dependent receptors.In most environments, the amount of free iron (approximately 10 Ϫ9 M) is below the concentration required by most microorganisms for growth (49). To fulfill their requirements for iron, bacteria have developed several strategies, including (i) the reduction of ferric to ferrous ions, (ii) the direct acquisition of iron from host proteins, (iii) the secretion of highaffinity iron-chelating compounds, called siderophores, and (iv) the uptake of heterologous siderophores (23). The transport of (heterologous) siderophores and heme/hemophores through the outer membranes of gram-negative bacteria is an energy-dependent process that needs the function of the ExbBExbD-TonB system. This system transduces the proton-motive force of the cytoplasmic membrane to the high-affinity outer membrane receptors, which are therefore also known as TonBdependent receptors (48). The synthesis of siderophores and their transport systems is negatively regulated by the Fur repressor protein (18). In addition, some siderophore transport systems are also positively regulated by the presence of their cognate siderophores (12). This level of regulation often involves extracytoplasmic function (ECF) sigma factors (56). The ECF family of sigma factors constitutes a group of environmentally responsive transcriptional factors of the RpoD ( 70 ) family that control a wide range of bacterial functions (39). The ECF factors involved in the regulation of iron uptake systems have been classified as the iron starvation class of sigma factors (35). One well-studied example is ferric dicitrate uptake by Escherichia coli (10

Iron acquisition in the cystic fibrosis lung and potential for novel therapeutic strategies

“…P. aeruginosa strains have w30 genes encoding TonB-dependent receptors (TBDRs), which are involved in the uptake of ferrisiderophores (Cornelis & Bodilis, 2009). Receptors for mycobactin and carboxymycobactin (Llamas et al, 2008), ferrichrome and ferrioxamine (Llamas et al, 2006;Cuív et al, 2007;Hannauer et al, 2010), rhizobactin, aerobactin and schizokinen (Cuív et al, 2006), and vibriobactin (Elias et al, 2011) have all been identified. Given the evidence to date, P. aeruginosa appears to have the greatest capacity for siderophore piracy in the CF lung microbiome.…”

Citrate-mediated iron uptake in Pseudomonas aeruginosa: involvement of the citrate-inducible FecA receptor and the FeoB ferrous iron transporter

“…citrate (Cox, 1980b;Harding & Royt, 1990)]. While outer-membrane receptors for most of these heterologous siderophores have been identified (Banin et al, 2005;Cuiv et al, 2006;Ghysels et al, 2005;Llamas et al, 2006Llamas et al, , 2008Poole et al, 1990), additional (e.g. periplasmic or inner-membrane) transport components are generally not evident from the genome sequence.…”

“…P. aeruginosa produces two siderophores, pyoverdine (Meyer & Hornsperger, 1978;Poole et al, 1991;Visca et al, 2007) and pyochelin Heinrichs et al, 1991;, though the organism is also able to utilize several siderophores produced by fungi and other bacteria (Cornelis et al, 1987;Cuiv et al, 2006Cuiv et al, , 2007Llamas et al, 2006Llamas et al, , 2008Meyer, 1992;Meyer et al, 1999;Poole et al, 1990), as well as additional molecules with Fe-chelating ability [e.g. citrate (Cox, 1980b;Harding & Royt, 1990)].…”

FvbA is required for vibriobactin utilization in Pseudomonas aeruginosa

“…It has been shown that P. aeruginosa has two receptors that mediate the uptake of the E. coli siderophore enterobactin, PfeA and PirA (Dean & Poole, 1993;Ghysels et al, 2005). In addition, P. aeruginosa is capable of utilizing the Mycobacterium paratuberculosis siderophore mycobactin through FemA (Llamas et al, 2008), ferrichrome through FiuA, ferrioxamine through FoxA (Llamas et al, 2006), citrate through FecA (Cox, 1980), aerobactin through ChtA (Cuív et al, 2006), and the siderophores cepabactin (Mislin et al, 2006), desferrichrysin, desferricrocine and coprogen (Meyer, 1992) through unidentified receptors. It is interesting to note that the above-mentioned receptors probably represent only a part of the total iron-acquisition potential of P. aeruginosa.…”

“…The expression of the TonB-dependent siderophore receptor is tightly regulated by the level of iron (Noinaj et al, 2010). Furthermore, in several cases, the siderophore itself acts as a signal and induces the expression of its cognate receptor (Beare et al, 2003;Llamas et al, 2006;Neilands, 1995). The binding of the siderophore to its receptor induces the expression of various genes, including genes required for siderophore uptake, and can be regulated through several regulatory pathways.…”