Insights into the mechanism regulating the differential expression of the P28-OMP outer membrane proteins in obligatory intracellular pathogen <i>Ehrlichia chaffeensis</i>
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<i>Ehrlichia chaffeensis</i> causes human monocytic ehrlichiosis (HME), which is one of the most prevalent, life-threatening emerging infectious zoonoses. The life cycle of <i>E. chaffeensis</i> includes ticks and mammals, in which <i>E. chaffeensis</i> proteins are expressed differentially contributing to bacterial survival and infection. Among the <i>E. chaffeensis</i> P28-OMP outer membrane proteins, OMP-1B and P28 are predominantly expressed in tick cells and mammalian macrophages, respectively. The mechanisms regulating this differential expression have not been comprehensively studied. Here, we demonstrate that the transcriptional regulators EcxR and Tr1 regulate the differential expression of <i>omp-1B</i> and <i>p28</i> in <i>E. chaffeensis.</i> Recombinant <i>E. chaffeensis</i> Tr1 bound to the promoters of <i>omp-1B</i> and <i>p28,</i> and transactivated <i>omp-1B</i> and <i>p28</i> promoter-EGFP fusion constructs in <i>Escherichia coli</i>. The consensus sequence of Tr1 binding motifs was A<sup>C</sup>/<sub>T</sub>TATA as determined with DNase I footprint assay. Tr1 showed a higher affinity towards the <i>p28</i> promoter than the <i>omp-1B</i> promoter as determined with surface plasmon resonance. EcxR activated the <i>tr1</i> expression in response to a temperature decrease. At 37°C low level of Tr1 activated the <i>p28</i> expression. At 25°C high level of Tr1 activated the <i>omp-1B</i> expression, while repressing the <i>p28</i> expression by binding to an additional site upstream of the <i>p28</i> gene. Our data provide insights into a novel mechanism mediated by Tr1 regulating <i>E. chaffeensis</i> differential gene expression, which may aid in the development of new therapeutics for HME.
