Evidence that the N-terminal region of the Vibrio fischeri LuxR protein constitutes an autoinducer-binding domain

The Vibrio fischeri luminescence genes are regulated by the LuxR protein and an N-acyl homoserine lactone compound termed the autoinducer. The C-terminal one-third of LuxR contains a domain that can interact with the transcription complex and activate the luminescence genes. On the basis of limited evidence it has been suggested that the N-terminal two-thirds of LuxR constitutes a domain that serves to bind the autoinducer. We show that tritium-labeled autoinducer binds to Escherichia coli cells in which LuxR is overexpressed. We also show that tritium-labeled autoinducer binds to E. coli in which truncated LuxR proteins missing portions of the C-terminal domain are expressed but does not bind to E. coli cells in which truncated LuxR proteins missing portions of the N-terminal region are expressed. Our results provide evidence that the autoinducer binds to LuxR and that in E. coli the N-terminal two-thirds of LuxR can fold into a polypeptide capable of binding the autoinducer in the absence of the C-terminal domain.