Structural and Biosynthetic Analysis of the Fabrubactins, Unusual Siderophores from Agrobacterium fabrum Strain C58

Abstract

Siderophores are iron-chelating molecules produced by microorganisms and plants to acquire exogenous iron. Siderophore biosynthetic enzymology often produces elaborate and unique molecules through unusual reactions to enable specific recognition by the producing organisms. Herein, we report the structure of two siderophore analogs from Agrobacterium fabrum strain C58, which we named fabrubactin (FBN) A and FBN B. Additionally, we characterized the substrate specificities of the NRPS and PKS components. The structures suggest unique Favorskii-like rearrangements of the molecular backbone that we propose are catalyzed by the flavin-dependent monooxygenase, FbnE. FBN A and B contain a 1,1-dimethyl-3-amino-1,2,3,4-tetrahydro-7,8-dihydroxy-quinolin (Dmaq) moiety previously seen only in the anachelin cyanobacterial siderophores. We provide evidence that Dmaq is derived from l-DOPA and propose a mechanism for the formation of the mature Dmaq moiety. Our bioinformatic analyses suggest that FBN A and B and the anachelins belong to a large and diverse siderophore family widespread throughout the Rhizobium/Agrobacterium group, α-proteobacteria, and cyanobacteria.

Automated genome mining predicts combinatorial diversity and taxonomic distribution of peptide metallophore structures

“…32,33 The quinoline chelator Dmaq was first identified in anachelins, 34 and the biosynthesis was recently established for fabrubactins (Figure 1). 35 Synthesis is initialized by FbnL and FbnM , which form a two-protein heme peroxidase that oxidizes L-tyrosine to L-DOPA. 35 GrbDE and FbnLM were previously used as a handle for genome mining, 32,35 and our pHMMs gave similar results.…”

“…35 Synthesis is initialized by FbnL and FbnM , which form a two-protein heme peroxidase that oxidizes L-tyrosine to L-DOPA. 35 GrbDE and FbnLM were previously used as a handle for genome mining, 32,35 and our pHMMs gave similar results. The diverse pyoverdine family of siderophores is defined by a fluorescent chelating chromophore (Figure 1).…”

“…Additionally, one rare pathway was not included: fabrubactins contain two α-hydroxycarboxylate chelating moieties not reported in any other metallophore (Figure 1A, bolded atoms). 35 Both substructures are proposed to be synthesized by the flavin-dependent monooxygenase FbnE. A profile HMM was built for FbnE (Supplemental dataset) but was not included in antiSMASH, as all BGCs with FbnE hits were either captured independently by FbnL and FbnM (Dmaq biosynthesis), or appeared to be false positives based on a lack of other metallophore-related genes.…”