Molecular cloning and expression of ptxA, the gene encoding the 120-kilodalton cytotoxin of Actinobacillus pleuropneumoniae serotype 2.

The genetic determinants of the 120-kDa cytotoxin of Actinobacillus pleuropneumoniae serotype 2 were isolated from a lambda DNA library by a plaque immunoblot technique. Expression of the 120-kDa polypeptide was confirmed by Western immunoblot analysis of infected Escherichia coli cell lysates, which were shown to be toxic for porcine alveolar macrophages in vitro. The genetic determinants of the toxin were subcloned into the plasmid vector pUC18. This plasmid (pPTX1) directed the synthesis and secretion of the active 120-kDa cytotoxin in E. coli. The recombinant toxin was indistinguishable from native cytotoxin from A. pleuropneumoniae serotype 2 with respect to molecular size, reaction in Western blot analysis, heat lability, cytotoxic activity, and neutralization by serum antibody. A restriction endonuclease cleavage map of pPTX1 was prepared, and deletion mutants were used to locate the minimal region of DNA required for production of intracellular toxin; this gene was termed ptxA. Southern hybridization analysis with a 1.7-kb PvuII fragment located within the ptxA gene revealed sequences with a high degree of homology in serotype reference strains 2, 3, 4, 6, and 8. Other reference strains did not contain sequences that were recognized by this probe. However, related sequences (greater than 71% homology) were detected in Actinobacillus actinomycetemcomitans and A. equuli. Weak hybridization was observed between the ptxA probe and pLKT5, which carries the lktAC genes of Pasteurella haemolytica, and between the ptxA probe and pAPH1, which carries the structural gene for type II hemolysin from A. pleuropneumoniae. The isolation of the genetic determinants of this cytotoxin will enable investigations of the structure and organization of the ptx DNA region and further analysis of its role in the pathogenesis of pleuropneumonia.