At low cell density, in absence of autoinducer has a kinase activity, and autophosphorylates on a histidine residue. The phosphoryl group is then transferred to an aspartate residue in the response regulator domain. The phosphoryl group is transferred to LuxU, and ultimately to LuxO. At high cell density, in the presence of autoinducer, the kinase activity is inactivated, and the response regulator domain has a phosphatase activity (By similarity). ATP + protein L-histidine = ADP + protein N-phospho-L-histidine. Binds the complex formed by the autoinducer and LuxP.