Identification and characterization of an iron-regulated gene, chtA, required for the utilization of the xenosiderophores aerobactin, rhizobactin 1021 and schizokinen by Pseudomonas aeruginosa

Abstract

Pseudomonas aeruginosa utilizes several xenosiderophores under conditions of iron limitation, including the citrate hydroxamate siderophore aerobactin. Analysis of the P. aeruginosa genome sequence revealed the presence of two genes, chtA (PA4675) and PA1365, encoding proteins displaying significant similarity to the aerobactin outer-membrane receptor, IutA, of Escherichia coli. The chtA and PA1365 genes were mutated by insertional inactivation and it was demonstrated that ChtA is the outer-membrane receptor for aerobactin. ChtA also mediated the utilization of rhizobactin 1021 and schizokinen, which are structurally similar to aerobactin. In contrast to the utilization of other xenosiderophores by P. aeruginosa, there was no apparent redundancy in the utilization of aerobactin, rhizobactin 1021 and schizokinen. The utilization of citrate hydroxamate siderophores by P. aeruginosa was demonstrated to be TonB1 dependent. A Fur box was identified in the region directly upstream of chtA and it was demonstrated by the in vivo Fur titration assay that this region is capable of binding Fur and accordingly that expression of chtA is iron regulated. The PA1365 mutant was unaffected in the utilization of citrate hydroxamate siderophores. INTRODUCTIONWith few exceptions, all organisms display an absolute nutritional requirement for iron. For pathogenic bacteria, the host environment may be considered an iron-depleted environment, with all available iron located intracellularly or bound by high-affinity iron-binding proteins such as transferrin and lactoferrin (Litwin & Calderwood, 1993;Payne, 1993;Crosa, 1997). Under conditions of iron limitation, many bacteria produce low-molecular-mass highaffinity chelators termed siderophores that bind ferric iron and deliver it to the cell via specific outer-membrane receptors. Additionally, many bacterial pathogens also encode specific transport systems that allow for the assimilation of iron from host proteins such as haemoglobin, transferrin and lactoferrin (Ratledge & Dover, 2000;Genco & Dixon, 2001).Pseudomonas aeruginosa is an opportunistic human pathogen that causes severe and often fatal infections and is the predominant cause of respiratory infections in individuals with cystic fibrosis. P. aeruginosa synthesizes two known siderophores, pyoverdine (Cox & Adams, 1985) and pyochelin (Cox, 1980), and has also been shown to be capable of utilizing a variety of xenosiderophores including enterobactin (Poole et al., 1990) and desferrioxamine mesylate (Meyer, 1992). The ability of P. aeruginosa to utilize a wide variety of xenosiderophores possibly allows it to compete effectively with other micro-organisms for available iron and consequently to occupy extended ecological niches. However, despite the variety of xenosiderophores known to be transported by P. aeruginosa, in many cases their cognate outer-membrane receptors have not been identified.Siderophore receptors function as gated channels in the outer membrane and display relatively tight specificity for their cognate sideropho...

Identification and Characterization of an Outer Membrane Receptor Gene in <i>Acinetobacter baumannii</i> Required for Utilization of Desferricoprogen, Rhodotorulic Acid, and Desferrioxamine B as Xenosiderophores

“…Similarly, the ChtA receptor in Pseudomonas aeruginosa is able to recognize and transport citrate-containing hydroxamate-type xenosiderophores such as aerobactin, rhizobactin 1021, and schizokinen. 34) In any event, it should be noted that these receptors for multiple ferric siderophores contain the conserved amino acid residues in the TonB boxes involved in the interaction of these receptors with the TonBExbB-ExbD complex as the energy supplying system. 13,30) In contrast, little is known about multiple receptors specific to an inherent siderophore, with the exception of the type 1 ferric pyoverdine receptors, FpvA and FpvB, in P. aeruginosa 35) and the ferric vibrioferrin receptors, PvuA1 and PvuA2, in Vibrio parahaemolyticus.…”

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)].…”