Characterization of the BaeSR two-component system from Salmonella Typhimurium and its role in ciprofloxacin-induced mdtA expression

Two-component systems are one of the most prevalent mechanisms by which bacteria sense, respond and adapt to changes in their environment. The activation of a sensor histidine kinase leads to autophosphorylation of a conserved histidine residue followed by transfer of the phosphoryl group to a cognate response regulator in an aspartate residue. The search for antibiotics that inhibit molecular targets has led to study prokaryotic two-component systems. In this study, we characterized in vitro and in vivo the BaeSR two-component system from Salmonella Typhimurium and evaluated its role in mdtA regulation in response to ciprofloxacin treatment. We demonstrated in vitro that residue histidine 250 is essential for BaeS autophosphorylation and aspartic acid 61 for BaeR transphosphorylation. By real-time PCR, we showed that mdtA activation in the presence of ciprofloxacin depends on both members of this system and that histidine 250 of BaeS and aspartic acid 61 of BaeR are needed for this. Moreover, the mdtA expression is directly regulated by binding of BaeR at the promoter region, and this interaction is enhanced when the protein is phosphorylated. In agreement, a BaeR mutant unable to phosphorylate at aspartic acid 61 presents a lower affinity with the mdtA promoter.