Sequence determinants spanning -35 motif and AT-rich spacer region impacting Ehrlichia chaffeensis Sigma 70-dependent promoter activity of two differentially expressed p28 outer membrane protein genes

Evaluating EcxR for Its Possible Role in Ehrlichia chaffeensis Gene Regulation

Ehrlichia chaffeensis, a tick-transmitted intraphagosomal bacterium, is the causative agent of human monocytic ehrlichiosis. The pathogen also infects several other vertebrate hosts. E. chaffeensis has a biphasic developmental cycle during its growth in vertebrate monocytes/macrophages and invertebrate tick cells. Host- and vector-specific differences in the gene expression from many genes of E. chaffeensis are well documented. It is unclear how the organism regulates gene expression during its developmental cycle and for its adaptation to vertebrate and tick host cell environments. We previously mapped promoters of several E. chaffeensis genes which are recognized by its only two sigma factors: σ32 and σ70. In the current study, we investigated in assessing five predicted E. chaffeensis transcription regulators; EcxR, CtrA, MerR, HU and Tr1 for their possible roles in regulating the pathogen gene expression. Promoter segments of three genes each transcribed with the RNA polymerase containing σ70 (HU, P28-Omp14 and P28-Omp19) and σ32 (ClpB, DnaK and GroES/L) were evaluated by employing multiple independent molecular methods. We report that EcxR binds to all six promoters tested. Promoter-specific binding of EcxR to several gene promoters results in varying levels of gene expression enhancement. This is the first detailed molecular characterization of transcription regulators where we identified EcxR as a gene regulator having multiple promoter-specific interactions.

Insights into the mechanism regulating the differential expression of the P28-OMP outer membrane proteins in obligatory intracellular pathogen Ehrlichia chaffeensis

Ehrlichia chaffeensis causes human monocytic ehrlichiosis (HME), which is one of the most prevalent, life-threatening emerging infectious zoonoses. The life cycle of E. chaffeensis includes ticks and mammals, in which E. chaffeensis proteins are expressed differentially contributing to bacterial survival and infection. Among the E. chaffeensis 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 omp-1B and p28 in E. chaffeensis. Recombinant E. chaffeensis Tr1 bound to the promoters of omp-1B and p28, and transactivated omp-1B and p28 promoter-EGFP fusion constructs in Escherichia coli. The consensus sequence of Tr1 binding motifs was A^C/_TTATA as determined with DNase I footprint assay. Tr1 showed a higher affinity towards the p28 promoter than the omp-1B promoter as determined with surface plasmon resonance. EcxR activated the tr1 expression in response to a temperature decrease. At 37°C low level of Tr1 activated the p28 expression. At 25°C high level of Tr1 activated the omp-1B expression, while repressing the p28 expression by binding to an additional site upstream of the p28 gene. Our data provide insights into a novel mechanism mediated by Tr1 regulating E. chaffeensis differential gene expression, which may aid in the development of new therapeutics for HME.

Mutations in Ehrlichia chaffeensis Genes ECH_0660 and ECH_0665 Cause Transcriptional Changes in Response to Zinc or Iron Limitation

Ehrlichia chaffeensis causes human monocytic ehrlichiosis by replicating within phagosomes of monocytes/macrophages. A function disruption mutation within the pathogen's ECH_0660 gene, which encodes a phage head-to-tail connector protein, resulted in the rapid clearance of the pathogen in vivo, while aiding in induction of sufficient immunity in a host to protect against wild-type infection challenge. In this study, we describe the characterization of a cluster of seven genes spanning from ECH_0659 to ECH_0665, which contained four genes encoding bacterial phage proteins, including the ECH_0660 gene. Assessment of the promoter region upstream of the first gene of the seven genes (ECH_0659) in Escherichia coli demonstrated transcriptional enhancement under zinc and iron starvation conditions. Furthermore, transcription of the seven genes was significantly higher under zinc and iron starvation conditions for E. chaffeensis carrying a mutation in the ECH_0660 gene compared to the wild-type pathogen. In contrast, for the ECH_0665 gene mutant with the function disruption, transcription from the genes was mostly similar to that of the wild type or was moderately downregulated. Recently, we reported that this mutation caused a minimal impact on the pathogen's in vivo growth, as it persisted similarly to the wild type. The current study is the first to describe how zinc and iron contribute to E. chaffeensis biology. Specifically, we demonstrated that the functional disruption in the gene encoding the phage head-to-tail connector protein in E. chaffeensis results in the enhanced transcription of seven genes, including those encoding phage proteins, under zinc and iron limitation. IMPORTANCE Ehrlichia chaffeensis, a tick-transmitted bacterium, causes human monocytic ehrlichiosis by replicating within phagosomes of monocytes/macrophages. A function disruption mutation within the pathogen's gene encoding a phage head-to-tail connector protein resulted in the rapid clearance of the pathogen in vivo, while aiding in induction of sufficient immunity in a host to protect against wild-type infection challenge. In the current study, we investigated if the functional disruption in the phage head-to-tail connector protein gene caused transcriptional changes resulting from metal ion limitations. This is the first study describing how zinc and iron may contribute to E. chaffeensis replication.

Sequence Determinants Spanning -10 Motif and Spacer Region Implicated in Unique Ehrlichia chaffeensis Sigma 32-Dependent Promoter Activity of dnaK Gene

Ehrlichia chaffeensis is an obligate intracellular tick-borne bacterium that causes human monocytic ehrlichiosis. Studying Ehrlichia gene regulation is challenge, as this and related rickettsiales lack natural plasmids and mutagenesis experiments are of a limited scope. E. chaffeensis contains only two sigma factors, σ³² and σ⁷⁰. We previously developed Escherichia coli surrogate system to study transcriptional regulation from RNA polymerase (RNAP) containing Ehrlichia σ³² or σ⁷⁰. We reported that RNAP binding motifs of E. chaffeensis genes recognized by σ³² or σ⁷⁰ share extensive homology and that transcription may be initiated by either one of the sigma factors, although transcriptional efficiencies differ. In the current study, we investigated mapping the E. chaffeensis dnaK gene promoter using the pathogen σ³² expressed in E. coli lacking its native σ³². The E. coli surrogate system and our previously described in vitro transcription system aided in defining the unique −10 motif and spacer sequence of the dnaK promoter. We also mapped σ³² amino acids/domains engaged in its promoter regulation in E. chaffeensis. The data reported in this study demonstrate that the −10 and −35 motifs and spacer sequence located between the two motifs of dnaK promoter are critical for the RNAP function. Further, we mapped the importance of all six nucleotide positions of the −10 motif and identified critical determinants within it. In addition, we reported that the lack of C-rich sequence upstream to the −10 motif is unique in driving the pathogen-specific transcription by its σ³² from dnaK gene promoter. This is the first study in defining an E. chaffeensis σ³²-dependent promoter and it offers insights about how this and other related rickettsial pathogens regulate stress response genes.