Actinobacillus pleuropneumoniae iron transport: a set of exbBD genes is transcriptionally linked to the tbpB gene and required for utilization of transferrin-bound iron

Upon iron restriction, Actinobacillus pleuropneumoniae has been shown to ex press the transferrin-binding proteins TbpB and TbpA, both of which have be en implied to be important virulence factors. In order to identify addition al iron-regulated proteins, we cloned and analyzed the region upstream of t he transferrin-binding protein genes in an A. pleuropneumoniae serotype 7 s train. We located immediately upstream of the tbpB gene two open reading fr ames which were 43% homologous to the neisserial ExbBD protein genes. By ra ising specific antibodies, we showed that ExbB is expressed under iron-limi ting growth conditions only, and RT-PCR analysis revealed that the exbBD ge nes and the tbpB gene are transcribed on a single polycistronic mRNA. By co nstructing an isogenic and nonpolar exbBD mutant, we show-ed that the exbBD genes are required by A. pleuropneumoniae for utilization of transferrin-b ound iron, Using PCR and Western blotting, we showed that the genetic organ ization found in A. pleuropneumoniae serotype 7 is similar in all 12 A. ple uropneumoniae serotype reference strains.