Iron acquisition by Actinobacillus suis: identification and characterization of transferrin receptor proteins and encoding genes.

Actinobacillus suis is an important pathogen of swine, especially in high-health-status herds. A published report mentioning the binding of porcine transferrin (Tf) by at least one strain of A. suis suggested that A. suis, like other members of the Pasteurellaceae, can acquire Tf-bound iron by means of a siderophore-independent, receptor-mediated mechanism. The objective of the present study was to characterize the components involved in this process, if present. Growth assays, with seven strains, confirmed that A. suis can use porcine (but not human or bovine) Tf as an iron source for growth. In solid phase binding assays, total membranes derived from all strains exhibited strong binding of porcine Tf, but only if the membranes were from organisms grown under iron-restricted conditions. An affinity-isolation procedure allowed the isolation of putative Tf-binding polypeptides ( approximately 100 and approximately 63 kDa) from comparable membranes from all strains. PCR approaches allowed the amplification, cloning and sequencing of A. suis tonB, exbB, exbD, tbpB and tbpA homologues. Reverse transcription (RT)-PCR, using RNA from organisms grown under iron-replete and iron-restricted conditions, revealed that tonB, exbB, exbD, tbpB and tbpA are transcribed as a single unit with expression being up-regulated in response to iron restriction. The calculated molecular masses of the predicted, mature TbpA (104.3 kDa) and TbpB (63.4 kDa) proteins suggest strongly that the affinity-isolated, approximately 100 and approximately 63 kDa Tf-binding polypeptides represent TbpA and TbpB, respectively. It is concluded that the acquisition of Tf-bound iron by A. suis involves mechanisms analogous to those found in other members of the Pasteurellaceae.