Analysis of hemolysin operons in Actinobacillus pleuropneumoniae

Abstract

Among the twelve different serotypes of Actinobacillus pleuropneumoniae, the causative agent of swine pleuropneumonia, a strongly active hemolysin I (HlyI) is produced by serotypes which are particularly virulentand less active hemolysin II (HlyII) is produced by all serotypes except type 10. In the serotypes 1, 5a, 5b, 9, 10 and 11, which produce HlyI, the hemolysin ( hly) operon consists of a structural hlylA gene, encoding pre-HlyI, an activator gene, hlyIC, necessary for the activation of pre-Hly to active Hlyand two genes, hlyIB and hlyID, involved in Hly secretion. These genes are clustered in the order, hlyICABD. This is characteristic to RTX toxin (repeats in the structural toxin) operons. The HlyII operons in all serotypes producing HlyII consist only of the pre-HlyII-encoding gene, appAand its activator gene, appC. The serotypes, which produce HlyII, but not HlyI, contain a truncated HlyI operon, with the promoter, hlyIB and hlyIDand a small segment of the C terminus of hlylA. This partial HlyI operon might have been formed by deletion of hlyIC and most of hlylA. In serotype 3, which produces HlyII, but no HlyIand which releases only minute amounts of this Hly into the growth medium, none of the hlyI genes and consequently no Hly secretion genes were found. The above results postulate that HlyII is secreted via the products of hlyIB and hlyIDand explain the low amount of HlyII secreted by serotype 3. Cloning and analysis of the structural genes encoding pre-HlyI and pre-HlyII among the different serotypes revealed differences in the hlylA genes which are highly similar in the serologically related serotypes 1, 9 and 11and differ from the serotypes, 5a, 5b and 10. The hlyIIA genes, in contrast, seem to be conserved in all serotypes.