Characterization of the Achromobactin Iron Acquisition Operon in Sodalis glossinidius

Sodalis glossinidius is a facultative, extra- and intracellular symbiont found in most tissues of the tsetse fly (Glossinia sp.).Sodalis has a putative achromobactin siderophore iron acquisition system on the pSG1 plasmid. Reverse transcription (RT)-PCR analysis revealed that the achromobactin operon is transcribed as a single polycistronic molecule and is expressed whenSodalis is within the tsetse fly. Expression of the achromobactin operon was repressed under iron-replete conditions; in a mutant that lacks the iron-responsive transcriptional repressor protein Fur, expression was aberrantly derepressed under these iron-replete conditions, indicating that the Fur protein repressed achromobactin gene expression when iron was plentiful. A putative Fur binding site within theSodalis achromobactin promoter bound Fur inEscherichia coli Fur titration assays. Wild-typeSodalis produced detectable siderophorein vitro , but a mutation in the putative achromobactin biosynthesis geneacsD eliminated detectable siderophore production inSodalis . Reduced growth of the siderophore synthesis mutant was reconstituted by addition of exogenous achromobactin, suggesting the strain retains a functional siderophore transport system; however, reduced growth of aSodalis ferric-siderophore outer membrane receptor mutant with a mutation inacr was not reconstituted by exogenous siderophore due to its defective transporter. TheSodalis siderophore synthesis mutant showed reduced growth in tsetse that lacked endogenous symbionts (aposymbiotic) when the flies were inoculated withSodalis intrathoracically, but not when inoculatedper os . Our findings suggest thatSodalis siderophores play a role in iron acquisition in certain tsetse fly tissues and provide evidence for the regulation of iron acquisition mechanisms in insect symbionts.