At low cell density, in the absence of AI-1 (autoinducer 1), LuxN has a kinase activity and autophosphorylates on His-471. The phosphoryl group is then transferred on Asp-771 of the response regulator domain. The phosphoryl group is transferred to LuxU, and ultimately to LuxO. At high cell density, in the presence of AI-1, the kinase activity is inactivated, and the response regulator domain has a phosphatase activity. LuxN phosphatase acts on itself. As LuxU could function to establish an equilibrium between the aspartyl-phosphate of LuxN and the aspartyl-phosphate of LuxO, LuxU transfers phosphate from LuxO to LuxN and finally phosphate is drained from the system (By similarity). ATP + protein L-histidine = ADP + protein N-phospho-L-histidine.