Regulation of gene expression by the HlyX protein of Actinobacillus pleuropneumoniae

Abstract

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 Afnr 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 i t s 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 @(frdA'-lacZ) in E. coli JRG1787 (Afnr); 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.

Enzymes Involved in Anaerobic Respiration Appear To Play a Role inActinobacillus pleuropneumoniaeVirulence

“…The suggested influence of other factors coregulating the expression of HlyX-dependent genes is supported by the work of Soltes and MacInnes (42), who showed that HlyX-dependent expression of a frdA-lacZ fusion in E. coli varied depending on growth phase and carbon source. Therefore, we hypothesize that as-yet-unknown factors in BALF are responsible for upregulation of aspartase activity and transcription of aspA.…”

Molecular cloning and characterization of the ferric hydroxamate uptake (fhu) operon in Actinobacillus pleuropneumoniae The GenBank accession number for the sequence of the fhuCDBA operon of Actinobacillus pleuropneumoniae serotype 1 reference strain 4074 described in this study is AF351135.

“…All of these factors may contribute to the virulence of this bacterium. A. pleuropneumoniae also responds to ironrestricted conditions by inducing the synthesis of a specific subset of outer-membrane proteins (OMPs) (Deneer & Potter, 1989 ;Niven et al, 1989 ;Soltes & MacInnes, 1994 ; M. Archambault, personal communication), including two membrane-bound transferrinspecific receptors called TbpA and TbpB (Gerlach et al, 1992a, b ;Gonzalez et al, 1995). Although it was tempting to speculate that one of these proteins might serve as a receptor for a siderophore, preliminary bioassays by Deneer & Potter (1989) did not demonstrate any siderophore production in A. pleuropneumoniae.…”

Deletion of the Anaerobic Regulator HlyX Causes Reduced Colonization and Persistence ofActinobacillus pleuropneumoniaein the Porcine Respiratory Tract

“…An A. pleuropneumoniae FNR homologue, HlyX, has been found to induce hemolytic activity in E. coli under anoxic conditions and to be able to complement E. coli fnr deletions (18,26). Like FNR, HlyX contains four iron-sulfur clusters responsible for the DNA-binding ability of the protein (12).…”