The cyanobacteria are photosynthetic procaryotes that carry out an oxygen-evolving photosynthesis virtually identical to that of higher plants. Some cyanobacteria are also capable of nitrogen fixation. Because the enzyme that performs nitrogen fixation is extremely sensitive to oxygen, nitrogen fixation must occur in an anaerobic environment. this creates a special problem for cyanobacteria since oxygen is generated as a byproduct of photosynthesis, and yet photosynthesis is required to produce the energy for nitrogen fixation. Certain filamentous cyanobacteria such as Anabaena deal with this problem by physically separating the functions of nitrogen fixation and photosynthesis. Nitrogen starvation of Anabaena induces certain vegetative cells along the filament to develop into specialized cells called heterocysts. Heterocysts are the exclusive sites of nitrogen fixation within the filament and carry out an incomplete photosynthesis relative to neighboring vegetative cells.

We are interested in how heterocyst development is regulated, in particular how gene expression is controlled during differentiation. Our work has focused on the regulation of gene expression at the level of transcription. One longstanding project concerns characterization of the sequences required to promote gene transcription in Anabaena and how these sequences are utilized for differential gene expression in different cell types during development. We are utilizing a large set of well characterized photosynthesis and nitrogen metabolism genes whose products are differentially expressed in heterocyst and vegetative cells as tools in this project. Using promoter-reporter gene fusions assayed in vivo, we are defining the location of promoter sequences, studying the role of individual nucleotides within promoter regions, examining cell-specific expression, and looking at whether promoter switching occurs during heterocyst development. Additional projects involve the identification and characterization of genes that are up-regulated during heterocyst development.