Citryl Ornithine Is an Intermediate in a Three-Step Biosynthetic Pathway for Rhizoferrin in Francisella

The Gram-negative bacterium Francisella tularensis secretes the siderophore rhizoferrin to scavenge necessary iron from the environment. Rhizoferrin, also produced by a variety of fungi and bacteria, comprises two citrate molecules linked by amide bonds to a central putrescine (diaminobutane) moiety. Genetic analysis has determined that rhizoferrin production in F. tularensis requires two enzymes: FslA, a siderophore synthetase of the nonribosomal peptide synthetase-independent siderophore synthetase (NIS) family, and FslC, a pyridoxal-phosphate-dependent decarboxylase. To discern the steps in the biosynthetic pathway, we tested F. tularensis strain LVS and its ΔfslA and ΔfslC mutants for the ability to incorporate potential precursors into rhizoferrin. Unlike putrescine supplementation, supplementation with ornithine greatly enhanced siderophore production by LVS. Radioactivity from L-[U-¹⁴C] ornithine, but not from L-[1-¹⁴C] ornithine, was efficiently incorporated into rhizoferrin by LVS. Although neither the ΔfslA nor the ΔfslC mutant produced rhizoferrin, a putative siderophore intermediate labeled by both [U-¹⁴C] ornithine and [1-¹⁴C] ornithine was secreted by the ΔfslC mutant. Rhizoferrin was identified by liquid chromatography and mass spectrometry in LVS culture supernatants, while citryl-ornithine was detected as the siderophore intermediate in the culture supernatant of the ΔfslC mutant. Our findings support a three-step pathway for rhizoferrin production in Francisella; unlike the fungus Rhizopus delemar, where putrescine functions as a primary precursor for rhizoferrin, biosynthesis in Francisella preferentially starts with ornithine as the substrate for FslA-mediated condensation with citrate. Decarboxylation of this citryl ornithine intermediate by FslC is necessary for a second condensation reaction with citrate to produce rhizoferrin.