LuxR is the core component of Vibrio fischeri quorum sensing. It acts as the transcriptional activator by binding to its cognate signaling molecules 3-oxo-hexanoyl-homoserine lactone (3OC6HSL). Although several acyl-HSLs with 3-oxo groups are known to activate LuxR with similar efficiency, acyl-HSLs without 3-oxo groups are very weak inducers. We conducted a round of LuxR directed evolution to acquire LuxR mutants with higher signal sensitivity to octanoylhomoserine lactone (C8HSL). All of the isolated mutants showed increased signal sensitivity to many other acyl-HSLs, including C8HSL, and some to the LuxR antagonist p-coumaroyl-HSL. The evolution of their ligand sensitivity proceeded through the stabilization of the signal-bound state, thereby elevating the effective concentration of LuxR at the ON-state. (C) 2016, The Society for Biotechnology, Japan. All rights reserved.