Francisella tularensis is unusual among Gram-negative bacteria in that its genome does not encode orthologs for TonB, ExbB and ExbD that typically energize the uptake of iron across the outer membrane. This organism secretes however a siderophore similar in structure to rhizoferrin. The fsl operon of six genes encodes functions for biosynthesis and uptake of the siderophore. Two of these genes encode a siderophore synthetase belonging to the nonribosomal peptide synthetase (NRPS)-independent synthetase (NIS)-family and a protein belonging to the pyridoxyl phosphate-dependent decarboxylase family, and both are required for siderophore production. Siderophore utilization involves the product of the fslE gene, a protein unique to Francisella species that could function as a siderophore receptor. Additionally, genes related in sequence to fslE also play a role in siderophore acquisition. The mechanism for TonB-independent iron uptake in this microorganism remains to be elucidated.

Further reading: Iron Uptake and Homeostasis in Microorganisms

Labels: Francisella, Iron acquisition mechanisms, Iron transporters, Iron uptake in Francisella, Iron uptake systems, Iron-homeostasis, Iron-metabolism, Iron-uptake