Identification of a cytolethal distending toxin gene locus and features ofa virulence-associated region in Actinobacillus actinomycetemcomitans

A genetic locus for a cytolethal distending toxin (CDT was identified in a polymorphic region of the chromosome of Actinobacillus actinomycetemcomitan s, a predominant oral pathogen, The locus was comprised of three open readi ng frames (ORFs) that had significant amino acid sequence similarity and mo re than 90% sequence identity to the cdtABC genes of some pathogenic Escher ichia coli strains and Haemophilus ducreyi, respectively. Sonic extracts fr om recombinant E. coli, containing the A. actinomycetemcomitans ORFs, cause d the distension and killing of Chinese hamster ovary cells characteristic of a CDT, Monoclonal antibodies made reactive with the CdtA, CdtB, and CdtC proteins of H, ducreyi recognized the corresponding gene products from the recombinant strain. CDT-like activities were no longer expressed by the re combinant strain when an Omega Kan-2 interposon was inserted into the cdtA and cdtB genes. Expression of the CDT-like activities in A. actinomycetemco mitans was strain specific. Naturally occurring expression-negative strains had large deletions within the region of the cdt locus. The cdtABC genes w ere flanked by an ORF (virulence plasmid protein), a partial ORF (integrase ), and DNA sequences (bacteriophage integration site) characteristic of vir ulence-associated regions. These results provide evidence fora functional C DT in a human oral pathogen.