ACQUISITION IN Erwinia chrysanthemi
An analysis of the role of Fe in pathogenicity requires a characterization of the high-affinity Fe uptake systems produced by the pathogen in Fe-limited environments. E. chrysanthemi 3937 synthesizes two structurally unrelated siderophores, achromobactin and chrysobactin (Figure 10-1), which are produced in a sequential manner in culture supernatants of bacterial cells grown under Fe limitation (Franza et al., 2005). In low-Fe cultures, achromobactin can be detected during the mid-exponential phase of growth while chrysobactin appears much later. As a catecholate readily identifiable using the Arnow-test, chrysobactin was the first siderophore of strain 3937 to be characterized (Persmark et al., 1989). Achromobactin is a hydroxycarboxylate siderophore that does not contain any catecholate or hydroxamate group (Munzinger et al., 2000). This siderophore was uncovered in chrysobactin deficient mutants, which are still able to form a halo of discoloration on CAS agar medium, a universal assay for siderophore detection (Schwyn and Neilands, 1987). Chrysobactin-deficient mutants fail to grow in the presence of the ferric Fe chelator EDDHA, but the production of achromobactin enables them to thrive on a medium containing the ferrous Fe chelator 2,2' dipyridyl.
Structural properties of chrysobactin have been studied in detail (Persmark et al., 1989). Chrysobactin, identified as dihydroxybenzoyl)-D-lysyl]-L-serine, belongs to a class of siderophores which are basically dihydroxybenzoic acid (DHB) derivatives of amino acids or peptides. Unlike the tricatecholate siderophore enterobactin and other hexadentate ligands, which are strong Fe binders, chrysobactin possesses only three potential coordination sites for complexing ferric Fe, two hydroxyl groups on the catechol moiety and the terminal carboxylate group of serine. Persmark and Neilands (1992) have shown that only catecholate hydroxyl groups are involved in the chelation suggesting that chrysobactin is a bidentate ligand, and thus a relatively weak ligand. However, depending on the pH and metal/ligand concentration ratio, chrysobactin was found to form ferric complexes of different stoichiometries, from 1:1 to 1:3 (Fe:chrysobactin). When ligand is four or more times in excess, there is a mixture of bis and tris complexes in solution at physiological pH. To further understand the biological function of chrysobactin, Albrecht-Gary and coworkers have investigated the coordination properties of the different ferric complexes of this siderophore. They found that chrysobactin is a less effective ferric chelator than hexadentate siderophores, such as enterobactin or ferrioxamine B. However, chrysobactin exhibits higher pFe value than citrate or malate (pFe of chrysobactin = 17.1, vs. pFe of citrate = 14.8), which are known to be the major ferric carriers in plants, and can effectively sequester Fe from their ferric complexes (Albrecht - Gary, personal communication, September 2, 2002). Achromobactin belongs to a class of siderophores that involves carboxylate or/and hydroxy donor groups for Fe binding. Although the coordination chemistry of this ligand has not yet been investigated, it can be predicted that achromobactin is also competitive in the environment in which it is meant to function.
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