Engineering Pseudochelin Production in Myxococcus xanthus.

Myxobacteria utilize the catechol natural products myxochelin A and B in order to maintain their iron homeostasis. Recently, the production of these siderophores, along with a new myxochelin derivative named pseudochelin A, was reported for the marine bacterium S2040. The latter derivative features a characteristic imidazoline moiety, which was proposed to originate from an intramolecular condensation reaction of the β-aminoethyl amide group in myxochelin B. To identify the enzyme catalyzing this conversion, we compared the myxochelin regulons of two myxobacterial strains that produce solely myxochelin A and B with those of S2040. This approach revealed a gene exclusive to the myxochelin regulon in S2040, coding for an enzyme of the amidohydrolase superfamily. To prove that this enzyme is indeed responsible for the postulated conversion, the reaction was reconstituted using a hexahistidine-tagged recombinant protein made in , with myxochelin B as the substrate. To test the production of pseudochelin A under conditions, the amidohydrolase gene was cloned into the myxobacterial plasmid pZJY156 and placed under the control of a copper-inducible promoter. The resulting vector was introduced into the myxobacterium DSM 16526, a native producer of myxochelin A and B. Following induction with copper, the myxobacterial expression strain was found to synthesize small quantities of pseudochelin A. Replacement of the copper-inducible promoter with the constitutive promoter led to increased production levels in , which facilitated the isolation and subsequent structural verification of the heterologously produced compound. In this study, an enzyme for imidazoline formation in pseudochelin biosynthesis was identified. Evidence for the involvement of this enzyme in the postulated reaction was obtained after reconstitution. Furthermore, the function of this enzyme was demonstrated by transferring the corresponding gene into the bacterium , which thereby became a producer of pseudochelin A. In addition to clarifying the molecular basis of imidazoline formation in siderophore biosynthesis, we describe the heterologous expression of a gene in a myxobacterium without chromosomal integration. Due to its metabolic proficiency, represents an interesting alternative to established host systems for the reconstitution and manipulation of biosynthetic pathways. Since the plasmid used in this study is easily adaptable for the expression of other enzymes as well, we expand the conventional expression strategy for myxobacteria, which is based on the integration of biosynthetic genes into the host genome.