Part of the two-component regulatory system SenX3/RegX3. Phosphorylates RegX3 under conditions of phosphate limitation. Probably does not itself sense phosphate concentrations, which may be relayed to SenX3 by the PstSCAB phosphate transporter system. ATP + protein L-histidine = ADP + protein N-phospho-L-histidine. By phosphate-limiting growth conditions. Extended N-terminus.