A novel siderophore-independent strategy of iron uptake in the genus Burkholderia

Like many other bacteria, Burkholderia sp. take up iron in its ferric form via siderophore-dependent transporters. We observed that mutant strains of B.cenocepaciaH111 unable to synthesize siderophores did not exhibit any growth defect under iron limited conditions. This finding suggested that this opportunistic pathogen can adopt an alternative iron uptake strategy to compensate for the loss of siderophores. We identified a putative iron uptake locus, ftr(Bcc)ABCD, in the genome of B.cenocepaciaH111, which is also conserved in other members of the genus Burkholderia. Mutants deficient in both siderophore-dependent and Ftr(Bcc)ABCD systems failed to grow under iron-limited conditions and radiolabelled iron transport assays showed that these mutants were impaired in iron uptake. In addition, expression of ftr(Bcc)ABCD can restore growth of an E.coli strain lacking all known high-affinity iron transport systems under iron-limited conditions. We show that all four proteins encoded by ftr(Bcc)ABCD are essential for iron uptake. Furthermore, our results indicate that the expression of ftr(Bcc)ABCD is regulated at the transcriptional level by iron concentration. This study provides evidence of an alternative, siderophore-independent, iron uptake system in Burkholderia species.