The Actinobacillus pleuropneumoniae hemolysin determinant: unlinked appCA and appBD loci flanked by pseudogenes

The appBD genes encoding the secretion functions for the 110-kDa RTX hemolysin of Actinobacillus pleuropneumoniae have been cloned and sequenced. Unlike analogous genes from other RTX determinants, the appBD genes do not lie immediately downstream from the hemolysin structural gene, appA. Although isolated from a diverse group of gram-negative organisms, the appBD genes and the characterized RTX BD genes from other organisms all exhibit a high degree of homology at both the DNA and predicted amino acid sequence levels. Analysis of the DNA sequences 3' to appA and 5' to appB suggests that these regions harbor remnant RTX B and A pseudogenes, respectively. Although the appA gene is most similar to the lktA gene from Pasteurella haemolytica (Y. F. Chang, R. Young, and D. K. Struck, DNA 8:635-647, 1989), the RTX A pseudogene upstream from appB most closely resembles the hlyB gene from Escherichia coli, suggesting that the appCA and appBD operons were derived from different ancestral RTX determinants.