Member of the two-component regulatory system CitA/CitB. Probably activates CitB by phosphorylation. The periplasmic domain binds H-citrate(2-), which is essential for induction of the citrate-fermentation genes. ATP + protein L-histidine = ADP + protein N-phospho-L-histidine. Homodimer. In vitro CitB and the CitA kinase domain form a complex, formation of which is enhanced by ATP. Autophosphorylated.