Molecular characterization of staphyloferrin B biosynthesis in Staphylococcus aureus

Siderophores are iron‐scavenging molecules produced by many microbes. In general, they are synthesized using either non‐ribosomal peptide synthetase (NRPS) or NRPS‐independent siderophore (NIS) pathways. Staphylococcus aureus produces siderophores, of which the structures of staphyloferrin A and staphyloferrin B are known. Recently, the NIS biosynthetic pathway for staphyloferrin A was characterized. Here we show that, in S. aureus, the previously identified sbn (siderophore biosynthesis) locus encodes enzymes required for the synthesis of staphyloferrin B, an α‐hydroxycarboxylate siderophore comprised of l‐2,3‐diaminopropionic acid, citric acid, 1,2‐diaminoethane and α‐ketoglutaric acid. Staphyloferrin B NIS biosynthesis was recapitulated in vitro, using purified recombinant Sbn enzymes and the component substrates. In vitro synthesized staphyloferrin B readily promoted the growth of iron‐starved S. aureus, via the ABC transporter SirABC. The SbnCEF synthetases and a decarboxylase, SbnH, were necessary and sufficient to produce staphyloferrin B in reactions containing component substrates l‐2,3‐diaminopropionic acid, citric acid and α‐ketoglutaric acid. Since 1,2‐diaminoethane was not required, this component of the siderophore arises from the SbnH‐dependent decarboxylation of a 2,3‐diaminoproprionic acid‐containing intermediate. Liquid chromatography‐electrospray ionization‐mass spectrometry (LC‐ESI‐MS) analyses of a series of enzyme reactions identified mass ions corresponding to biosynthetic intermediates, allowing for the first proposed biosynthetic pathway for staphyloferrin B.