Phosphorelay protein which receives sensory signals from LuxN and LuxP and transmits them to LuxO, at low cell density. LuxN and LuxP transfer a phosphoryl group to LuxU on His-58 and this phosphoryl group is further transferred to LuxO. At high cell density, 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 probably LuxP) and finally phosphate is drained from the system. Monomer.