UC Santa Barbara

Iron is essential to nearly all forms of life, but is predominantly insoluble under aerobic conditions at physiological pH, reducing its bioavailability. Microbes have adapted to low-iron environments by producing siderophores, small Fe(III)-chelating natural products, as a means for acquiring iron. Siderophores typically contain catecholate, hydroxamate, or a-hydroxycarboxylate groups to coordinate iron. This work investigates an emerging class of Fe(III)-siderophore ligands, the C-diazeniumdiolate group. Work presented herein provides insight into the biosynthesis and photoreactivity of the C-diazeniumdiolate group, expanding our knowledge of this new class of molecules. Natural products containing N-N bond linkages have attracted much interest, particularly in terms of their biosynthetic precursor substrates, including the C-diazeniumdiolate amino acid graminine (Gra). Gra can be incorporated into peptidic siderophores by non-ribosomal peptide synthetases (NRPS). In this work, a genome-mining approach aided the discovery of three new Gra-containing siderophores, isolated and characterized from a marine-derived strain, Tistrella mobilis KA081020-065, (tistrellabactins A and B) and a lake sediment-derived strain, Pandoraea norimbergensis DSM 11628 (pandorachelin). Tistrellabactins A and B arise from a promiscuous NRPS adenylation domain, which can accept both L-Asn and L-Asp substrates. The N-N bond is a distinguishing feature of Gra, yet the origin and reactivity of the C-diazeniumdiolate group has remained elusive until now. Through isotopic labeling, the direct precursor of L-Gra is identified as L-Arg. Furthermore, the N-N bond in Gra is derived from the Nd and Nw of the guanidinium group of L-Arg. Gra-containing siderophores are photoreactive upon UV irradiation. In this work, the photoproduct of Gra-containing siderophores is characterized as E/Z oxime isomers. The unique chemistry of the C-diazeniumdiolate group brings about the question of whether these siderophores may serve a function beyond iron acquisition.