STRUCTURAL-ANALYSIS OF THE ACTINOBACILLUS-PLEUROPNEUMONIAE-RTX-TOXIN-I (APXI) OPERON

Actinobacillus pleuropneumoniae-RTX-toxin I (ApxI), an important virul ence factor, is secreted by serotypes 1, 5, 9, 10, and 11 of A. pleuro pneumoniae. However, sequences homologous to the secretion genes apxIB D of the ApxI operon are present in all 12 serotypes except serotype 3 . The purpose of this study was to determine and compare the structure s of the ApxI operons of the 12 A. pleuropneumoniae serotypes. We focu sed on the nucleotide sequence comparison of the ApxI-coding genes, th e structures of the ApxI operons, and the transcription of the ApxI op erons. We determined the nucleotide sequences of the toxin-encoding ap xICA genes of serotype 9 and found that the gene for the structural to xin, apxI4, was almost identical to the apxI4 gene of serotype 1. The toxin-encoding genes of the other serotypes are also similar for the m ain part; nevertheless, two variants were identified, one in serotypes 1, 9, and 11 and one in serotypes 5 and 10. The two apxI4 variants di ffer mainly within the distal 110 nucleotides. Structural analysis dem onstrated that intact ApxI operons, consisting of the four contiguous genes apxICABD, are present in serotypes 1, 5, 9, 10, and 11. ApxI ope rons with a major deletion in the apxICA genes are present in serotype s 2, 4, 6, 7, 8, and 12. Serotype 3 does not contain ApxI operon seque nces. We found that all ApxI operons are transcriptionally active desp ite the partial deletion of the operon in some serotypes. The implicat ions of these data for the expression and secretion of ApxI and the ot her Apx-toxins, ApxII and ApxIII, as well as for the development of a subunit vaccine against A. pleuropneumoniae will be discussed.