Regulation of gene expression by the HlyX protein of Actinobacillus pleuropneumoniae.

The hlyX gene of the swine pathogen Actinobacillus pleuropneumoniae and the fnr gene of Escherichia coli encode very similar proteins. The hlyX gene is able to complement delta fnr mutations and will permit the growth of E. coli fnr strains in nitrate minimal salts medium under anoxic conditions. In addition, the hlyX gene product appears to induce the expression of a latent haemolytic activity as evidenced by the presence of a strong zone of haemolysis around E. coli (hlyX+) colonies grown on bovine or ovine blood. In this study, the ability of the hlyX gene product to induce haemolytic activity and regulate expression of frdA and its own gene was examined in the presence of various carbon sources and in the presence and absence of iron; fnr was included for comparison. The HlyX protein was able to induce the synthesis of the latent E. coli haemolytic activity only under anoxic conditions. Haemolytic activity was highest during the late exponential phase and then levelled off in the stationary phase. The hlyX gene product was able to activate the expression of a phi (frdA'-lacZ) in E. coli JRG1787 (delta fnr); however, the level of expression depended on carbon source, growth phase and copy number. Like fnr, the hlyX gene product appeared to affect its own synthesis but the nature and extent of regulation depended not only on the presence of oxygen but also on growth conditions.