A conserved residue in the histidine kinase BceS tunes bacitracin stress responses in Bacillus subtilis

Two-component systems regulate bacterial responses through histidine kinases (HKs), where the majority act as both a kinase and a phosphatase. In Bacillus subtilis , the BceRS-BceAB system mediates bacitracin resistance, but it is unknown whether its HK, BceS, also functions as a phosphatase. Here, we identify the conserved motif within the BceS DHp domain and show that Thr-128 contributes to maintaining kinase–phosphatase balance. Substitutions at this site impaired BceS signalling and bacitracin resistance. Our findings demonstrate that in BceS, Thr-128 is important for proper BceAB regulation.