At low cell density, in absence of AI-2 (autoinducer 2), LuxQ 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 AI-2, the kinase activity is inactivated, and the response regulator domain has a phosphatase activity. ATP + protein L-histidine = ADP + protein N-phospho-L-histidine. Binds the complex formed by AI-2 and LuxP.