Amplified expression of a transcriptional pattern formed during development of Anabaena

The cyanobacterium Anabaena responds to nitrogen deprivation by producing heterocysts, cells specialized for nitrogen fixation, at well-spaced intervals along its filaments. The gene hepA, required for heterocyst maturation, is expressed in response to nitrogen deprivation, prior to visible differentiation. A spatial pattern of hepA expression indistinguishable from the eventual pattern of heterocysts was made visible by fusing the hepA promoter to luxAB, which encodes bacterial luciferase. Because the resulting signal did not greatly exceed instrumental background, T7 RNA polymerase was used to increase luminescence. The hepA promoter was fused to the gene for that polymerase, and a promoter recognized by that polymerase was fused to luxAB. Filaments containing these two fusions showed spaced luminescing cells many hours before differentiation became discernible morphologically.