A Widely Distributed Bacterial Pathway for Siderophore Biosynthesis Independent of Nonribosomal Peptide Synthetases

Iron is an essential nutrient for virtually all microorganisms because it is a cofactor for several electron-transport proteins involved in vital life processes like aerobic and anaerobic ATP biosynthesis. However, the bioavailability of iron, which exists predominantly in its ferric form in aerobic environments, such as soil, is very low despite the fact that iron is the fourth most abundant element in the Earth’s crust. This is because, at neutral and alkaline pH, ferric iron forms insoluble, polymeric oxyhydroxide complexes that cannot be assimilated by microorganisms. Consequently, iron acquisition from the environment poses a significant challenge to saprophytic microorganisms. Similar bioavailability problems exist in the intercellular matrices of higher eucaryotes, where ferric iron is tightly bound to solubilising transport and storage glycoproteins, such as transferrin and lactoferrin. Thus, iron assimilation by invading pathogens, which is considered essential for establishing infection, also poses a significant challenge. A common strategy used by many pathogenic and saprophytic microorganisms to tackle the problem of low iron bioavailability is the biosynthesis and excretion of high-affinity iron chelators known as siderophores. 2] Once an excreted siderophore has scavenged ferric iron from the environment or host, the resulting iron–siderophore complex is readsorbed by bacterial cells by a membrane-associated ATP-dependent transport system that often exhibits high substrate selectivity. In fungi, the readsorption of iron–siderophore complexes is mediated by the siderophore iron transport (SIT) family of the major facilitator protein superfamily. Several different mechanisms have been proposed for the recovery of ferric iron from the siderophore complex and reduction to the ferrous form for storage and utilisation. 5] Many siderophores are polypeptides that are biosynthesised by members of the nonribosomal peptide synthetase (NRPS) multienzyme family, which is also responsible for the biosynthesis of the majority of microbial peptide antibiotics. The enzymology of NRPS-catalysed siderophore biosynthesis has been intensively studied over the last decade, and the biosynthetic mechanisms for several types of structurally diverse peptide siderophore are now well understood. On the other hand, several bacterial siderophores are not polypetides, but are assembled instead from alternating dicarboxylic acid and diamine or amino alcohol building blocks (which are nevertheless derived from amino acids) linked by amide or ester bonds. Examples include aerobactin (1), rhizobactin 1021 (2), achromobactin (3), vibrioferrin (4), 11] alcaligin (5), and desferrioxamine E (6). Pioneering biochemical genetic studies in the 1980s by Neilands and co-workers established that aero-