A first approach based on cloning and complementation tests using E. coli mutants defective in enterobactin production allowed the genetic region involved in chrysobactin-mediated Fe transport to be identified. Using a collection of mutants that were unable to grow in the presence of EDDHA, it was possible to characterize the different stages required for biosynthesis of chrysobactin and the transport back to the cell of its ferric complex (Enard et al., 1988). The genetic organisation of the chrysobactin system is reminiscent of that of the enterobactin region in Escherichia coli K12. It comprises four operons divergently transcribed from two bidirectional promoters (Figure 10-2). The fct-cbsCEBA operon encodes the receptor Fct and the enzymes leading to the DHB moiety in chrysobactin biosynthesis (Franza and Expert, 1991). A second operon, cbsHF, encodes chrysobactin synthase CbsF, which is predicted to be a nonribosomal peptide synthetase with a multimodular structure allowing the assemblage of the three components, DHB, D-lysine and L-serine (Expert et al., 2004). The CbsH protein is an oligopeptidase that degrades chrysobactin in the cytosol. Removal of this enzyme by mutation results in bacterial growth inhibition caused by intracellular ferric Fe chelation (Rauscher et al., 2002). Thus, this peptidase plays an important role in the control of Fe homeostasis in E. chrysanthemi. A third cluster of genes located upstream of the cbsHF operon encodes the four components of ferric chrysobactin permease (CbuBCDG) and a protein (P43) homologous to E. coli EntS, predicted to be involved in the excretion of chrysobactin to the external medium (Expert et al., 2004).
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