TheAcinetobacter baumannii8399 clinical isolate secretes dihydroxybenzoic acid (DHBA) and a high-affinity catechol siderophore, which is different from other bacterial iron chelators already characterized. Complementation assays with enterobactin-deficientEscherichia colistrains led to the isolation of a cosmid clone containingA. baumannii8399 genes required for the biosynthesis and activation of DHBA. Accordingly, the cloned fragment harbours adhbACEBpolycistronic operon encoding predicted proteins highly similar to several bacterial proteins required for DHBA biosynthesis from chorismic acid. Genes encoding deduced proteins related to theE. coliFes and theBacillus subtilisDhbF proteins, and a putativeYersinia pestisphosphopantetheinyl transferase, all of them involved in the assembly and utilization of catechol siderophores in other bacteria, were found next to thedhbACEBlocus. ThisA. baumannii8399 gene cluster also contained theom73,p45andp114predicted genes encoding proteins potentially involved in transport of ferric siderophore complexes. The deduced products of thep114andp45genes are putative membrane proteins that belong to the RND and MFS efflux pump proteins, respectively. Interestingly, P45 is highly related to theE. coliP43 (EntS) protein that participates in the secretion of enterobactin. Although P114 is similar to other bacterial efflux pump proteins involved in antibiotic resistance, its genetic arrangement within thisA. baumannii8399 locus is different from that described in other bacteria. The product ofom73is a Fur- and iron-regulated surface-exposed outer-membrane protein. These characteristics together with the presence of a predicted TonB box and its high similarity to other siderophore receptors indicate that OM73 plays such a role inA. baumannii8399. The 184 ntom73–p114intergenic region contains promoter elements that could drive the expression of these divergently transcribed genes, all of which are in close proximity to almost perfect Fur boxes. This arrangement explains the iron- and Fur-regulated expression ofom73, and provides strong evidence for a similar regulation for the expression ofp114.