ABSTRACT
Hemolysin genes of the reference strains of Actinobacillus pleuropneumoniae serotypes 1 and 2 were identified, cloned, and expressed in Escherichia coli by using polymerase chain reaction amplification with oligonucleotides derived from the DNA sequence of the corresponding appA gene from A. pleuropneumoniae serotype 5. The three genes from serotypes 1, 2, and 5 have identical restriction maps and appear to encode a hemolysin which was previously identified in serotype 2 and designated HlyII. Gene appA is different from hlyIA encoding the major hemolysin type I (HlyI) which was identified earlier in serotype 1. Polymerase chain reaction amplification with oligonucleotides derived from the DNA sequence of hlyIA of serotype 1 showed that the gene encoding HlyI is present in serotype 1 but not in serotype 2, in contrast to the gene encoding HlyII that was present in both serotypes. This was confirmed by Western blot (immunoblot) experiments using monoclonal antibodies specific for either recombinant HlyI or recombinant HlyII, which showed that A. pleuropneumoniae serotype 1 strain 4074 produces both HlyI and HlyII, whereas serotype 2 strain S1536 produces only HlyII. The expression of both hemolysins was investigated in all serotypes by the use of monoclonal antibodies. HlyI was shown to be expressed by the reference strains of serotypes 1, 5a, 5b, 9, 10, and 11, whereas HlyII was shown to be expressed by the reference strains of all 12 serotypes tested except serotype 10. A. pleuropneumoniae serotype 1 strain 4074 is the first bacterium which has been shown to contain two different actively expressed RTX toxin genes. Comparison of our data with those from other groups shows that the originally described strongly hemolytic hemolysin type I (HlyI) corresponds to cytolysin I (ClyI) which was recently described by others, while the weakly hemolytic hemolysin type II (HlyII) seems to be identical to ClyII and AppA.
