Acquisition of haemoglobin-bound iron by Histophilus somni

Histophilus somni is an important pathogen of cattle and sheep. H. somni requires iron and can use ruminant transferrins as iron sources for growth. Here, we investigated the abilities of bovine (strains 649 and 2,336) and ovine (strains 9L and 3384Y) isolates of H. somni to acquire iron from haemoglobins. Using growth assays, the bovine isolates were shown to acquire iron from bovine haemoglobin, but not from ovine, porcine or human haemoglobins; the ovine isolates, however, failed to use any of these haemoglobins as iron sources for growth. In solid phase binding assays, the bovine isolates, grown under iron-restricted conditions in the presence of bovine haemoglobin, bound not only bovine but also ovine and human haemoglobins. Competition binding assays indicated that all three haemoglobins were bound by the same receptor(s) and SDS-PAGE of membrane fractions revealed that expression of haemoglobin-binding activity was associated with the production of an approximately 120-kDa outer membrane protein. PCR approaches allowed the amplification and sequencing of hgbA, and also hugX and hugZ homologues from strains 649, 9L and 3384Y. While hgbA of strain 649 was predicted to encode an HgbA precursor that is processed to yield a mature, 123.9-kDa haemoglobin-binding protein, the hgbA genes of strains 9L and 3384Y were predicted to give rise to truncated products. RT-PCR experiments revealed that in strain 649, hugX, hugZ and hgbA are co-transcribed and iron-regulated and additional sequencing suggested that in strain 2336, expression of HgbA is subject to phase variation involving a poly C tract within hgbA.