Escherichia coli that carry a recombinant plasmid bearing the Vibrio fischeri lux regulon express luminescence that mimics the luminescence of V. fischeri. The lux regulon consists of two divergently transcribed operons, the rightward operon (luxICDABE genes) and the leftward operon (luxR gene). The luxR and luxI genes and the control region separating the two operons supply the primary regulatory control over the lux regulon; the regulatory mechanisms result in a dramatic increase in the rate of luciferase synthesis after induction, apparently due to a unique autoregulatory positive feedback mechanism, and in an enormous difference (greater than 10(4] in levels of luminescence in cells before and after induction. The generally accepted model of primary regulation of bioluminescence in V. fischeri involves the interaction of the product of the luxR gene and N-(3-oxohexanoyl)homoserine lactone, the autoinducer produced by the enzyme encoded by luxI, the first gene of the rightward operon, with an operator sequence within the control region to stimulate transcription of the rightward operon in a positive feedback loop. We have used deletion mapping of a transcription reporter vector to determine the approximate location of the operator. By site-directed mutagenesis of the presumed operator, we have demonstrated that the 20-base-pair inverted repeat ACCTGTAGGA/TCGTA CAGGT (where the vertical line is the center of symmetry), which bears striking similarity to the recognition sequence for the pleiotropic repressor protein LexA, is the operator of the lux regulon. We also found that deletion of sequences upstream of the palindrome leads to increased transcription from the rightward promoter (PR), indicative of a cis-acting element that represses transcription in the absence of the LuxR-autoinducer complex. Modifications of the palindrome that eliminate stimulation by LuxR-autoinducer of transcription from PR have no effect on repression by the cis-acting mechanism(s), suggesting that the palindrome is not necessary for repression of the rightward operon. Thus, it appears that the large increase in transcription upon induction of the lux regulon is the result of at least two independent mechanisms, one positive and the other negative.