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Haemophilus somnus Possesses Two Systems for Acquisition of Transferrin-Bound Iron

Andrew Ekins, Fariborz Bahrami, Ada Sijercic, Deborah Maret, and Donald F. Niven^*

Microbiology Unit, Department of Natural Resource Sciences, Macdonald Campus, McGill University, Sainte Anne de Bellevue, Quebec, Canada H9X 3V9

Received 31 October 2003/ Accepted 2 April 2004

Haemophilus somnus strain 649 was found to acquire iron from ovine, bovine, and goat transferrins (Tfs). Expression of Tf receptors, as evaluated by solid-phase binding assays, required the organisms to be grown under iron-restricted conditions in the presence of Tf. Competition binding assays revealed the presence of two distinct Tf-binding receptor systems, one specific for bovine Tf and the other capable of binding all three ruminant Tfs. Affinity isolation procedures using total membranes yielded three putative bovine Tf-binding polypeptides and one putative ovine and goat Tf-binding polypeptide. PCR amplification followed by DNA sequence analyses revealed that H. somnus strain 649 possesses genes that encode a bipartite TbpA-TbpB receptor along with a homolog of the Histophilus ovis single-component TbpA receptor. Expression of TbpB and the single-component TbpA would appear to be subject to a form of phase variation involving homopolymeric nucleotide tracts within the structural genes.

Present address: Department of Microbiology and Infectious Diseases, University of Calgary, Calgary, AB, Canada T2N 4N1.

Present address: Brain Tumour Research Centre, Montreal Neurological Institute, McGill University, Montreal, Quebec, Canada H3A 2B4.

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