We study the molecular genetics of nitrogen fixation and photosynthesis in cyanobacteria and purple bacteria. We also study genes encoding the enzyme acetyl-CoA carboxylase in plants, parasites, and mammals. The cyanobacterium Anabaena grows in filaments of 100 cells or more. When starved for nitrogen, specialized cells called heterocysts differentiate from the photosynthetic vegetative cells at regular intervals along each filament. Heterocysts are anaerobic factories for nitrogen fixation; in them, the nitrogenase enzyme complex is synthesized and the components of the oxygen-evolving photosystem II are turned off. More than 1000 genes are believed to be differentially expressed during the (irreversible) development of a heterocyst from a vegetative cell. We have cloned and sequenced genes for nitrogen fixation (nif) and others encoding RuBP carboxylase, glutamine synthetase, the D1, CP-47 and water-oxidizing proteins of photosystem II, all the components of phycobilisome rods, and the sigma and core sub-units of RNA polymerase. Mutants unable to fix nitrogen aerobically have been isolated. Among these are some that have altered heterocyst morphology or an altered pattern. Four of these have been studied in detail, using a complementation system to isolate the wild-type gene defective in the mutants. One mutant fails to deposit the necessary glycolipid layer that forms part of the heterocyst envelope. A second mutant fails to make any heterocysts at all. A third makes them only at the ends of filaments. A fourth makes them too late and too frequently! In these cases, the sequences of the complementing genes are highly informative, corresponding to proteins that participate in environment-sensing regulatory cascades.