Member of the two-component regulatory system FlgR/FlgS that induces the transcriptional induction of the genes needed in motility and flagellar biogenesis (PubMed:9882675). Also plays an essential role in bacterial survival at pH 2.5 independently of FlgR (PubMed:18978046). Functions as a sensor protein kinase which is autophosphorylated at a histidine residue and transfers its phosphate group to the conserved aspartic acid residue in the regulatory domain of FlgR (PubMed:10735847, PubMed:16035242). In turn, FlgR functions as a transcriptional regulator initiating transcription from RpoN-dependent promoters (PubMed:10735847, PubMed:16035242). ATP + protein L-histidine = ADP + protein N-phospho-L-histidine. Interacts (via its C-terminal kinase domain) with FlhA (via N-terminus). Autophosphorylated. Shows a pronounced survival defect at pH 2.5 not due to the nonmotile phenotype.