Member of the two-component regulatory system RstB/RstA. RstB functions as a membrane-associated protein kinase that phosphorylates RstA (Probable). ATP + protein L-histidine = ADP + protein N-phospho-L-histidine. Induced by low extracellular levels of magnesium via the PhoQ/PhoP two-component regulatory system. Autophosphorylated.