REGULATION OF GENE-EXPRESSION BY THE HLYX PROTEIN OF ACTINOBACILLUS-PLEUROPNEUMONIAE

The hlyX gene of the swine pathogen Actinobacillus pleuropneumoniae an d the fnr gene of Escherichia coil encode very similar proteins. The h lyX gene is able to complement Delta fnr mutations and will permit the growth of E. coli fnr strains in nitrate minimal salts medium under a noxic conditions. In addition, the hlyX gene product appears to induce the expression of a latent haemolytic activity as evidenced by the pr esence of a strong zone of haemolysis around E. coil (hlyX(+)) colonie s grown on bovine or ovine blood. In this study, the ability of the hl yX gene product to induce haemolytic activity and regulate expression of frdA and its own gene was examined in the presence of various carbo n sources and in the presence and absence of iron; fnr was included fo r comparison. The HlyX protein was able to induce the synthesis of the latent E. coil haemolytic activity only under anoxic conditions. Haem olytic 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 (De lta 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 d epended not only on the presence of oxygen but also on growth conditio ns.