The gene of the swine pathogen and the gene of encode very similar proteins. The gene is able to complement Δ mutations and will permit the growth of strains in nitrate minimal salts medium under anoxic conditions. In addition, the gene product appears to induce the expression of a latent haemolytic activity as evidenced by the presence of a strong zone of haemolysis around ( ) colonies grown on bovine or ovine blood. In this study, the ability of the gene product to induce haemolytic activity and regulate expression of and its own gene was examined in the presence of various carbon sources and in the presence and absence of iron; was included for comparison. The HlyX protein was able to induce the synthesis of the latent haemolytic activity only under anoxic conditions. Haemolytic activity was highest during the late exponential phase and then levelled off in the stationary phase. The gene product was able to activate the expression of a ø () in JRG1787 (Δ); however, the level of expression depended on carbon source, growth phase and copy number. Like , the 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.


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