CHARACTERIZATION OF THE HGBA LOCUS ENCODING A HEMOGLOBIN RECEPTOR FROM HAEMOPHILUS-DUCREYI

Haemophilus ducreyi can bind hemoglobin and use it as a source of heme , for which it has an obligate requirement. We previously identified a nd purified HgbA, a hemoglobin-binding outer membrane protein from H. ducreyi. In this report, we describe the molecular cloning, expression , DNA sequence, and mutagenesis of the structural gene for HgbA, hg6A, H. ducreyi and recombinant Escherichia coli expressing hgbA bound [I- 125]hemoglobin, establishing HgbA as a receptor. Insertions or deletio ns in the cloned hgbA gene abolished expression of HgbA and hemoglobin binding in E. coli, Mutagenesis of H. ducreyi by allelic exchange of insertions into hgbA abolished its ability to bind [I-125]hemoglobin o r utilize hemoglobin as a source of heme. The deduced protein sequence was similar to those of the TonB-dependent family of outer membrane r eceptors. The most similar member was HutA (heme receptor) from Vibrio cholerae. Tbp1 and Lbp1 (transferrin and lactoferrin receptors, respe ctively, from pathogenic Neisseria spp.) also showed very significant homology. Thus, by characterizing the hgbA locus, this work elucidates a potentially important role of HgbA in obtaining heme and/or iron fr om the host.