from Alexander J. Ninfa writing in Two-Component Systems in Bacteria:
Many of the two-component system transmitter proteins bring about both the phosphorylation and dephosphorylation of their cognate receiver proteins. While the mechanism of the transmitter protein kinase activity was relatively easy to discern biochemically, investigation of the phosphatase activity has proven more difficult owing to the involvement of widely dispersed portions of the transmitter protein in the activity and the corresponding requirement to study the activity using intact, full-length transmitter proteins. Here, I review the genetics, physiology, and biochemistry of the transmitter protein phosphatase activity, with special emphasis on possible mechanisms of receiver protein dephosphorylation, relationship of the transmitter phosphatase activity to the autophosphatase activity of the receiver protein, and physiological significance of the activity.
Further reading: Two-Component Systems in Bacteria