Glutathione Synthesis Regulated by CtrA Protects Ehrlichia chaffeensis From Host Cell Oxidative Stress.

Ehrlichia chaffeensis, a small Gram-negative obligatory intracellular bacterium, infects human monocytes or macrophages, and causes human monocytic ehrlichiosis, one of the most prevalent, life-threatening emerging zoonoses. Reactive oxygen species are produced by the host immune cells in response to bacterial infections. The mechanisms exploited by E. chaffeensis to resist oxidative stress have not been comprehensively demonstrated. Here, we found that E. chaffeensis encodes two functional enzymes, GshA and GshB, to synthesize glutathione that confers E. chaffeensis the oxidative stress resistance, and that the expression of gshA and gshB is upregulated by CtrA, a global transcriptional regulator, upon oxidative stress. We found that in E. chaffeensis, the expression of gshA and gshB was upregulated upon oxidative stress using quantitative RT-PCR. Ehrlichia chaffeensis GshA or GshB restored the ability of Pseudomonas aeruginosa GshA or GshB mutant to cope with oxidative stress, respectively. Recombinant E. chaffeensis CtrA directly bound to the promoters of gshA and gshB, determined with electrophoretic mobility shift assay, and activated the expression of gshA and gshB determined with reporter assay. Peptide nucleic acid transfection of E. chaffeensis, which reduced the CtrA protein level, inhibited the oxidative stress-induced upregulation of gshA and gshB. Our findings provide insights into the function and regulation of the two enzymes critical for E. chaffeensis resistance to oxidative stress and may deepen our understanding of E. chaffeensis pathogenesis and adaptation in hosts.

Acetylation of the CspA family protein CspC controls the type III secretion system through translational regulation of exsA in Pseudomonas aeruginosa.

The ability to fine tune global gene expression in response to host environment is critical for the virulence of pathogenic bacteria. The host temperature is exploited by the bacteria as a cue for triggering virulence gene expression. However, little is known about the mechanism employed by Pseudomonas aeruginosa to response to host body temperature. CspA family proteins are RNA chaperones that modulate gene expression. Here we explored the functions of P. aeruginosa CspA family proteins and found that CspC (PA0456) controls the bacterial virulence. Combining transcriptomic analyses, RNA-immunoprecipitation and high-throughput sequencing (RIP-Seq), we demonstrated that CspC represses the type III secretion system (T3SS) by binding to the 5' untranslated region of the mRNA of exsA, which encodes the T3SS master regulatory protein. We further demonstrated that acetylation at K41 of the CspC reduces its affinity to nucleic acids. Shifting the culture temperature from 25°C to 37°C or infection of mouse lung increased the CspC acetylation, which derepressed the expression of the T3SS genes, resulting in elevated virulence. Overall, our results identified the regulatory targets of CspC and revealed a regulatory mechanism of the T3SS in response to temperature shift and host in vivo environment.

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 AC/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.