Analysis of Achromobactin Biosynthesis by Pseudomonas syringae pv. syringae B728a

Pseudomonas syringae pv. syringae B728a is known to produce the siderophore pyoverdine under iron-limited conditions. It has also been proposed that this pathovar has the ability to produce a second siderophore, achromobactin. Here we present genetic and biochemical evidence supporting the hypothesis that P. syringae pv. syringae B728a produces both of these siderophores. We show that strains unable to synthesize either pyoverdine or achromobactin are unable to grow under iron-limiting conditions, which is consistent with these two molecules being the only siderophores synthesized by P. syringae pv. syringae B728a. Enzymes associated with achromobactin biosynthesis were purified and analyzed for substrate recognition. We showed that AcsD, AcsA, and AcsC together are able to condense citrate, ethanolamine, 2,4-diaminobutyrate, and alpha-ketoglutarate into achromobactin. Replacement of ethanolamine with ethylene diamine or 1,3-diaminopropane in these reactions resulted in the formation of achromobactin analogs that were biologically active. This work provides insights into the biosynthetic steps in the formation of achromobactin and is the first in vitro reconstitution of achromobactin biosynthesis.