Enzymatic tailoring of ornithine in the biosynthesis of the rhizobium cyclic trihydroxamate siderophore vicibactin

To acquire iron, the N₂-fixing, symbiotic bacterium Rhizobium sp. produce the cyclic trihydroxamate siderophore vicibactin, containing a 30-membered trilactone scaffold. Herein we report the overproduction and purification of the six proteins VbsACGOLS in the bacterial host Escherichia coli and the reconstitution of the biosynthesis of vicibactin from primary metabolites. The flavoprotein VbsO acts as a pathway-initiating L-ornithine N⁵-hydroxylase, followed by VbsA, which transfers (R)-3-hydroxybutyryl- from the CoA thioester to N⁵-hydroxyornithine to yield N⁵-((R)-3-hydroxybutyryl)-N⁵-hydroxy-L-ornithine. VbsL is a PLP-dependent epimerase acting at C² of the 10 atom monomer unit. VbsS, a nonribosomal peptide synthetase free-standing module, then activates N⁵-((R)-3-hydroxybutyryl)-N⁵-hydroxy-D- ornithine as the AMP anhydride on the way to cyclotrimerization to the vicibactin scaffold. The last step, tris-acetylation of the C² amino group of desacetyl-D-vicibactin to the mature siderophore vicibactin, is catalyzed distributively by VbsC, using three molecules of acetyl-CoA.