Natural competence in strains of Actinobacillus pleuropneumoniae.

We have identified a highly transformable strain of Actinobacillus pleuropneumoniae whose competence is regulated by the competence-activator Sxy as in other Pasteurellaceae. Other strains were poorly transformable or nontransformable. The genomes of two poorly transformable strains contain intact sets of competence genes. Moreover, we show that the low competence of one of these strains is not due to an inability to induce sxy expression or to a defect in Sxy function, suggesting that some other component of the competence system is defective. Although the A. pleuropneumoniae sxy gene has only 24% identity to its Haemophilus influenzae homologue, both genes fully complemented an H. influenzae sxy knockout, demonstrating that Sxy function is conserved throughout the Pasteurellaceae.