Characterization of two genes encoding distinct transferrin-binding proteins in different Actinobacillus pleuropneumoniae isolates.

The gene encoding the Actinobacillus pleuropneumoniae serotype 1 transferrin-binding protein (tfbA) was cloned, and the carboxy-terminal 70% of the protein was expressed as an aggregate protein in Escherichia coli. The nucleotide sequences of the tfbA genes from A. pleuropneumoniae serotypes 7 (G.-F. Gerlach, C. Anderson, A. A. Potter, S. Klashinsky, and P. J. Willson, Infect. Immun. 60:892-898, 1992) and 1 were determined, and a comparison revealed that they had 65% sequence identity. The deduced amino acid sequences showed a sequence agreement of 55%, and both proteins possessed a lipoprotein-like signal sequence. The serotype 1 TfbA protein had a predicted molecular mass of 65 kDa, compared with 60 kDa for the serotype 7 TfbA protein, and both proteins were immunologically distinct as assessed in a competitive enzyme-linked immunosorbent assay. Southern hybridization and Western blot (immunoblot) analysis of the 13 A. pleuropneumoniae type strains revealed that serotypes 2, 3, 4, 8, 9, 10, and 11 encode and express a TfbA protein highly homologous to that of A. pleuropneumoniae serotype 7 whereas the TfbA proteins and the encoding genes of serotypes 6 and 12 were highly homologous to that found in A. pleuropneumoniae serotype 1. The tfbA genes of A. pleuropneumoniae serotypes 5A and 5B were recognized, under medium-stringency hybridization conditions, by the A. pleuropneumoniae serotype 1-derived tfbA probe, and the respective proteins were weakly reactive with the antibody raised against the A. pleuropneumoniae serotype 7 TfbA protein.