Nitrogen fixing cyanobacteria are photosynthetic micro organisms that gain their energy from sunlight, their carbon and their nitrogen from air, and their reductant from water. Oxygen derived from photo-oxidation of water is highly damaging to nitrogenase, the enzyme responsible for reduction of molecular nitrogen. So, these cyanobacteria had to develop strategies to reconcile two incompatible processes, oxygenic photosynthesis and N₂ fixation. Some genera (eg Anabaena) of multicellular cyanobacteria solved the problem by differentiation of about every 10th cell of a filament into a heterocyst; a cell specialized for the task of N2 fixation. Heterocysts develop from photosynthetically active, vegetative cells upon N-starvation. Prospective heterocysts loose the capacity to fix CO₂, form a special envelope that limits the entrance of O₂, and enhance their respiratory capacity. This allows the mature heterocyst to generate a micro aerobic environment suitable for nitrogenase functioning. Adjacent vegetative cells supply heterocysts with carbohydrates that are then oxidised to provide reductants required for N₂ fixation and respiration. In turn, heterocysts provide vegetative cells with the needed fixed nitrogen. The prokaryotic organisation of DNA, the relative short live cycle, and the existence of cell types with distinct functions make heterocyst-forming cyanobacteria not only suitable for the study of biological N₂ fixation but also for the elucidation of developmental processes like pattern formation and differentiation. Using transposon mutagenesis, several mutants of Anabaena PCC sp. 7120 could be obtained, which are arrested in different states of development of their heterocysts. The genes affected in those mutants could be cloned and characterized. We started the biochemical characterization of the proteins, encoded by those genes. Physiological studies and description of ultra-structure by electron microscopy help to elucidate their function in differentiation. By analysing further mutants, we hope to get a detailed impression of the genetic and biochemical processes, underlying this interesting process of prokaryotic development.

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