Recombinant E. coli that produce light were found in a clone library of hybrid plasmids containing DNA from the marine bacterium Vibrio fischeri. All luminescent clones had a 16 kb insert that encoded enzymatic activities for the light reaction as well as regulatory functions necessary for expression of the luminescence phenotype (Lux). Mutants generated by transposons Tn5 and mini-Mu were used to define Lux functions and to determine the genetic organization of the lux region. Regulatory and enzymatic functions were assigned to regions of two lux operons. With transcriptional fusions between the lacZ gene or transposon mini-Mu and the target gene, expression of lux operons could be measured in the absence of light production. The direction of transcription of lux operons was deduced from the orientation of mini-Mu insertions in the fusion plasmids. Induction of transcription of one lux operon required a function encoded by that operon (autoregulation). From these and other regulatory relationships, we propose a model for genetic control of light production.