Expression of the putative siderophore receptor gene bfrZ is controlled bythe extracytoplasmic-function sigma factor BupI in Bordetella bronchiseptica

A new gene from Bordetella bronchiseptica, bfrZ encoding a putative siderop hore receptor, was identified in a Fur-repressor titration assay. A bfrZ nu ll mutant was constructed by allelic exchange, The protein profile of this mutant is similar to that of the wild-type parent strain. The BfrZ(-).BfrZ( +) isogenic pair was tested for utilization of 132 different siderophores a s iron sources. None of these iron sources acted as a ligand for BfrZ. Tran slational bfrZ:phoA and transcriptional bfrZ::lacZ fusions mere introduced into the B. bronchiseptica bfrZ locus. No alkaline phosphatase or beta -gal actosidase activity was detected. Sequence analysis of the bfrZ upstream re gion revealed the presence of two tightly linked genes, bup1 and bupR. Both of these genes are located downstream from a Fur-binding sequence. BupI is homologous to Escherichia coli FecI and Pseudomonas putida PupI and belong s to the family of extracytoplasmic-function sigma factors invoiced in tran scription of genes with extracytoplasmic functions. BupR is homologous to t he FecR and PupR antisigma factors and is predicted to be localized in the inner membrane. Similar to the surface signaling receptors FecA and PupB, B frZ bears an N-terminal extension. We found that bfrZ is not transcribed wh en bupI and bupR are expressed at the same level. However, overexpression o f bupI from a multicopy plasmid triggers bfrZ transcription, and under thes e conditions BfrZ was detected in membrane fractions. By analogy with the F ecI-FecR-FecA and PupI-PupR-PupB systems, our data suggest that bfrZ expres sion is inducible by binding of the cognate ligand to BfrZ and transduction of a signal through the envelope.