hgpB, a gene encoding a second Haemophilus influenzae hemoglobin- and hemoglobin-haptoglobin-binding protein

Haemophilus influenzae requires heme for growth and can utilize both hemoglobin and hemoglobin-haptoglobin as heme sources. We previously identified a hemoglobin- and hemoglobin-haptoglobin-binding protein, HgpA, in H. influenzae HI689. Mutation of hgpA did not affect binding or utilization of either heme source. The hgpA mutant exhibited loss of a 120-kDa protein and increased expression of a 115-kDa protein. These data suggested that at least one other gene product is involved in binding of these heme sources by H. influenzae. A 3.2-kbp PCR product derived from HI689 was cloned. The nucleotide sequence indicated a separate, distinct gene with high homology to hgpA, which would encode a 115-kDa protein. Primers were designed for directional cloning of the structural gene in the correct reading frame. Sonicates of induced Escherichia coli harboring the cloned open reading frame bound both hemoglobin and hemoglobin-haptoglobin. An insertion/deletion mutant of H. influenzae at the newly identified locus, designated hgpB, was constructed. The 115-kDa protein was not detected in the mutant after affinity purification using biotinylated hemoglobin. An hgpA hgpB double-mutant strain exhibited a reduced ability to utilize hemoglobin-haptoglobin, although it was unaltered in the ability to utilize hemoglobin. Affinity isolation of hemoglobin-binding proteins from the double mutant resulted in isolation of an approximately 120-kDa protein. Internal peptide sequencing revealed this protein to be a third distinct protein, highly homologous to HgpA and HgpB. In summary a second hemoglobin- and hemoglobin-haptoglobin-binding protein of H. influenzae has been identified and characterized, and the presence of an additional protein of similar function has been revealed.