The genes luxCDABE from four luminescent bacteria suffice for light production in Escherichia coli [Meighen, Microbiol. Rev. 55 (1991) 123-142]. We have inserted these gene clusters between inverted polylinkers, and placed the resulting cassettes as reporters within derivatives of transposon Tn5. Anabaena sp. strain PCC 7120 was mutagenized with these transposons. The luminescence of all but the most highly self-luminescent resulting derivatives of Anabaena sp. was strongly dependent on exogenously added aldehyde. Thus, luminescence based on luxCDABE is multiplicatively limited by production of luciferase and aldehyde. No toxicity was observed over protracted periods of luminescence. By deletion, new cassettes were derived in which only the aldehyde biosynthetic genes, luxCD-E, remained intact. Transcription was localized at the single-cell level in strains of cyanobacteria bearing constitutively expressed Xenorhabdus luminescens luxCD-E on a plasmid and relatively weakly expressed, developmentally regulated luxAB from Vibrio spp. in the chromosome. The developmentally critical gene, hetR, was thereby shown to remain active in mature heterocysts.