Pst and Tat Systems Are Essential for Ehrlichia chaffeensis Intracellular Survival and Regulated by CtrA

Ehrlichia chaffeensis is an obligate intracellular Gram-negative bacterium that infects human monocytes and macrophages, causing human monocytic ehrlichiosis (HME). HME is an emerging and highly threatening tick-borne infectious disease. CtrA functions as the response regulator of the CckA/CtrA two-component regulatory system, which exhibits high sequence conservation among α-proteobacteria. As a global transcriptional regulator, CtrA regulates the expression of genes associated with E. chaffeensis infection and intracellular survival. Here, we identified consensus CtrA-binding motifs in the promoter regions of genes encoding the phosphate-specific transport (Pst) system and twin-arginine translocation (Tat) system. Using electrophoretic mobility shift assay and reporter assay, our experimental evidence demonstrated that CtrA directly binds to the promoter regions of these target genes and further activates the expression of the corresponding genes. Through complementation experiments, we found that the E. chaffeensis Pst and Tat systems are functional and play critical roles in bacterial swimming motility, biofilm formation, and oxidative stress resistance. Using peptide nucleic acid-mediated knockdown, we further confirmed that the Pst and Tat systems are essential for E. chaffeensis intracellular survival. Collectively, our data advance the current understanding of the function and regulation of the Pst and Tat systems, which are essential for E. chaffeensis intracellular survival, and E. chaffeensis pathogenesis and host adaptation.