Bartonella quintana, the agent of
trench fever and an etiologic agent of
bacillary angiomatosis, has an extraordinarily high
hemin requirement for growth compared to other bacterial pathogens. We previously identified the major
hemin receptor of the pathogen as a 30-kDa
surface protein, termed HbpA. This report describes four additional homologues that share approximately 48% amino acid sequence identity with hbpA. Three of the genes form a paralagous cluster, termed hbpCAB, whereas the other members, hbpD and hbpE, are unlinked. Secondary structure predictions and other evidence suggest that Hbp family members are beta-barrels located in the outer membrane and contain eight transmembrane domains plus four extracellular loops. Homologs from a variety of gram-negative pathogens were identified, including Bartonella henselae Pap31, Brucella Omp31, Agrobacterium tumefaciens Omp25, and neisserial
opacity proteins (Opa). Family members expressed in vitro-synthesized
proteins ranging from ca. 26.5 to 35.1 kDa, with the exception of HbpB, an approximately 55.9-kDa
protein whose respective gene has been disrupted by a approximately 510 GC-rich
element containing variable-number tandem repeats. Transcription analysis by quantitative
reverse transcriptase-PCR (RT-PCR) indicates that all family members are expressed under normal culture conditions, with hbpD and hbpB transcripts being the most abundant and the rarest, respectively. Mutagenesis of hbpA by allelic exchange produced a strain that exhibited an enhanced
hemin-binding phenotype relative to the parental strain, and analysis by quantitative RT-PCR showed elevated transcript levels for the other hbp family members, suggesting that compensatory expression occurs.