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

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.