Identification and transcriptional organization of a gene cluster involved in biosynthesis and transport of acinetobactin, a siderophore produced by Acinetobacter baumannii ATCC 19606T

Abstract

In order to assimilate iron, Acinetobacter baumannii ATCC 19606T produces a siderophore named acinetobactin (Ab) that is composed of equimolar quantities of 2,3-dihydroxybenzoic acid (DHBA), L-threonine and N-hydroxyhistamine. Application of the Fur titration assay system to A. baumannii genomic libraries, followed by further cloning of the regions surrounding the candidate genes, led to the identification of the Ab cluster, which harbours the genetic determinants necessary for the biosynthesis and transport of the siderophore. However, an entA homologue essential for DHBA biosynthesis was not found in this cluster. Functions of potential biosynthetic genes inferred by homology studies suggested that the precursors, DHBA, L-threonine and N-hydroxyhistamine, are linked in steps resembling those of bacterial non-ribosomal peptide synthesis to form Ab. Genes responsible for the two-step biosynthesis of N-hydroxyhistamine from histidine were also identified in this cluster. Their genetic organization suggests that five genes involved in the transport system of ferric Ab into the cell cytosol form an operon. Construction of disruptants of some selected genes followed by phenotypic analysis supported their predicted biological functions. Interestingly, three additional genes probably involved in the intracellular release of iron from ferric Ab and the secretion of nascent Ab are contained in this cluster. Primer extension and RT-PCR analyses suggested that the Ab cluster, which includes 18 genes, is organized in seven transcriptional units originating from respective Fur-regulated promoter-operator regions.

Role of the Carboxy Terminus of SecA in Iron Acquisition, Protein Translocation, and Virulence of the Bacterial Pathogen Acinetobacter baumannii

“…The production and secretion of acinetobactin by the 2010 SecA mutant (Fig. 4) also indicate that this mutant produces the BarA and BarB cytoplasmic membrane-associated proteins, which are predicted to be involved in acinetobactin secretion (11,36). Taken together, these results indicate that, with the exception of BauA, the acinetobactin secretion and transport proteins are translocated by a SecA-independent mechanism, which most likely involves the signal recognition particle system because of their membrane location (59).…”

“…5). Considering the protein composition and location of the acinetobactin ABC transporter (11,36), it is possible to speculate that the 2010 derivative also produces the BauC and BauD cytoplasmic-associated acinetobactin transport proteins. The production and secretion of acinetobactin by the 2010 SecA mutant (Fig.…”

“…Accordingly, the A. baumannii ATCC 19606 T type strain produces and uses acinetobactin, a low-molecular-weight ferric iron chelator, as well as uncharacterized hemin utilization systems to scavenge iron from the host (9). Acinetobactin is a catecholhydroxamate siderophore synthesized and potentially secreted by proteins encoded by genes located within the acinetobactin and entA gene clusters (10)(11)(12). After scavenging extracellular iron, ferric acinetobactin complexes are transported through the membranes and internalized into the bacterial cytoplasm by the BauDCEBA transport protein complex.…”

“…After scavenging extracellular iron, ferric acinetobactin complexes are transported through the membranes and internalized into the bacterial cytoplasm by the BauDCEBA transport protein complex. The BauA protein, which is crucial for full virulence, is the acinetobactin cell surface receptor that transports the ferric acinetobactin complexes across the outer membrane and delivers them to BauB, a cytoplasmic membrane protein that harbors a periplasmic iron-binding domain and promotes the internalization of ferric acinetobactin via the BauC, BauD, and BauE cytoplasmic membrane-associated ABC transporter complex (10,11). Thus, the successful utilization of ferric acinetobactin as an iron source by A. baumannii ATCC 19606 T depends on the production and proper export of these proteins.…”

Structure and Function of the PiuA and PirA Siderophore-Drug Receptors from Pseudomonas aeruginosa and Acinetobacter baumannii

“…A. baumannii harbors between 21 and 27 TonB-dependent receptors compared to 35 in P. aeruginosa (26). So far, only FhuE (27) and BauA (28), the receptor for the siderophore acinetobactin (29,30), have been characterized in A. baumannii.…”