Summary auto-generated
Researchers cloned the bfrA gene from Bordetella bronchiseptica, a pathogenic bacterium that infects the respiratory tract. Using TnphoA mutagenesis, they identified an 80 kDa outer-membrane protein encoded by bfrA that belongs to the TonB-dependent iron receptor family, showing highest sequence homology to ferric siderophore receptors like Cir, IrgA, and BfeA. The bfrA gene is uniquely present in B. bronchiseptica but absent from the closely related species B. pertussis and B. parapertussis. Expression of bfrA is tightly regulated by iron availability through a Fur-dependent repression mechanism, with a Fur-binding consensus sequence overlapping the -10 promoter element. Although BfrA's specific iron substrate remains unclear, the study discovered additional iron sources utilized by B. bronchiseptica and B. pertussis, including DHBS (2,3-dihydroxybenzoylserine), ferrichrome, desferrioxamine B, and haemin. Notably, DHBS utilization depends on the bfeA receptor, while the other iron sources are accessed through alternative, previously uncharacterized mechanisms independent of both bfeA and bfrA.
Key findings
- The bfrA gene encodes an 80 kDa iron-regulated outer-membrane protein specific to B. bronchiseptica that is homologous to TonB-dependent ferric siderophore receptors, particularly Cir and IrgA.
- The bfrA gene is absent from B. pertussis and B. parapertussis despite their close phylogenetic relationship, indicating species-specific iron uptake mechanisms.
- bfrA expression is repressed by iron availability through Fur-mediated regulation, demonstrated by constitutive expression in fur mutants and iron-responsive alkaline phosphatase activity.
- B. bronchiseptica and B. pertussis utilize multiple iron sources including DHBS (via BfeA), ferrichrome, desferrioxamine B, and haemin through bfrA- and bfeA-independent pathways.
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Abstract
The bfrA (Bordetella bronchiseptica ferric iron repressed outer- membrane protein) gene was cloned from Bordetella bronchiseptica by screening a library of TnphoA insertion mutants for iron-repressed fusions to phoA. The bfrA gene encoded an 80 kDa outer-membrane protein with a high level of amino acid sequence identity to several bacterial proteins belonging to the family of Ton B-dependent outer-membrane receptors. BfrA was especially homologous to Cir of Escherichia coli, IrgA of Vibrio cholerae and to three previously characterized ferric enterobactin receptors. DNA hybridization results indicated that bfrA was not present in other Bordetella species. Expression of the bfrA gene was induced by low iron availability from a promoter overlapped by a sequence resembling a consensus Fur-binding sequence, and bfrA expression was derepressed in a B. bronchiseptica fur mutant. Utilization of the Bordetella siderophore alcaligin and the exogenous siderophore enterobactin was unaffected in bfrA mutants. Upon attempting to find the specificity of BfrA, 2,3-dihydroxybenzoylserine (DHBS) was shown to be utilized in a bfeA (Bordetella ferric enterobactin receptor gene)-dependent manner by B. bronchiseptica and B. pertussis. In addition, the hydroxamate siderophores ferrichrome and desferrioxamine B, and the iron source haemin were shown to be utilized independently of bfeA and bfrA in B. bronchiseptica and B. pertussis.