The Vibrio fischeri luminescence genes are activated by an autoinducer and the 250-amino acid residue LuxR protein. To develop a general view of LuxR structure and function, a set of luxR 5'-deletion mutations was generated. Ten luxR mutant plasmids encoding active LuxR proteins with deletions ranging from residues 2-5 (delta 2-5) to residues 2-182 (delta 2-182) were studied. The degree of transcriptional activation of luminescence genes by the truncated LuxR proteins ranged from 0.01% to greater than 200% of the wild-type level. LuxR proteins with small deletions (up to delta 2-20) were active and remained autoinducer-dependent, LuxR proteins with deletions between residues 2-58 and 2-138 showed low activity and were not affected by autoinducer, and LuxR proteins with large deletions such as the delta 2-162 protein were highly active and autoinducer-independent. However, proteins with deletions equal to or greater than delta 2-20 were unable to autoregulate luxR. Our data indicate there is a C-terminal LuxR domain capable of functioning as a transcriptional activator. We suggest that an N-terminal region of LuxR starting between residues 20 and 58 and extending to the region of residues 138-162 masks the activator function of the C-terminal domain. Residues prior to position 20 are needed for autoregulatory function. Experiments showing that wild-type luxR is dominant over luxR genes coding the delta 2-58 through delta 2-138 proteins indicate the N-terminal arm masks lux DNA binding.