Characterization of the Achromobactin Iron Acquisition Operon in Sodalis glossinidius
ABSTRACTSodalis glossinidiusis a facultative, extra- and intracellular symbiont found in most tissues of the tsetse fly (Glossiniasp.).Sodalishas 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 whenSodalisis 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 theSodalisachromobactin promoter bound Fur inEscherichia coliFur titration assays. Wild-typeSodalisproduced detectable siderophorein vitro, but a mutation in the putative achromobactin biosynthesis geneacsDeliminated 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 aSodalisferric-siderophore outer membrane receptor mutant with a mutation inacrwas not reconstituted by exogenous siderophore due to its defective transporter. TheSodalissiderophore synthesis mutant showed reduced growth in tsetse that lacked endogenous symbionts (aposymbiotic) when the flies were inoculated withSodalisintrathoracically, but not when inoculatedper os. Our findings suggest thatSodalissiderophores play a role in iron acquisition in certain tsetse fly tissues and provide evidence for the regulation of iron acquisition mechanisms in insect symbionts.