The Vibrio fischeri luminescence (lux) genes are regulated by the 250-amino-acid-residue LuxR protein and a V. fischeri metabolite termed autoinducer. The V. fischeri lux regulon consists of two divergently transcribed units. Autoinducer and LuxR activate transcription of the luxICDABE operon and autoregulate the luxR transcriptional unit. LuxR proteins with C-terminal truncations of up to 40 amino acid residues coded by plasmids with luxR 3'-deletion mutations are functional in negative autoregulation as demonstrated by using a luxR::lacZ transcriptional fusion as a luxR promoter probe in Escherichia coli. The truncated LuxR proteins showed little or no ability to activate transcription of luxICDABE, as indicated by using luminescence as a sensitive indicator of promoter strength in E. coli. Besides having no detectable activity as positive regulators of luxICDABE, LuxR proteins with C-terminal truncations of more than 40 amino acid residues had reduced or no detectable activity as negative autoregulators. The results suggest that amino acid residues in LuxR prior to no. 211 are sufficient for lux DNA binding. Residues in the region of 211 to 250 constitute a C-terminal tail that appears to be involved in activation of luxICDABE transcription either by interacting physically with the transcription initiation complex or by affecting lux DNA in the vicinity of the promoter.