Organization and Transcription of Nitrogen Fixation Genes in the Cyanobacterium Anabaena

Abstract

The mechanisms for protection of nitrogen fixation enzymes from inactivation by oxygen appear to be as richly varied in the cyanobacteria as they are in the bacteria. An extra challenge to the cyanobacteria is provided by one property they all share: photosynthetic evolution of oxygen in the light. For many species of cyanobacteria, both unicellular and filamentous, this feature means that nitrogen fixation is principally a laboratory phenomenon, made possible by experimental inhibition of photosystem II and continuous removal of oxygen. Under these circumstances nif gene expression is formally regulated like that of Klebsiella; repressed by either oxygen or combined nitrogen (ammonia, nitrate, urea or amino acids). For other species, capable of differentiating heterocysts at regular intervals along filaments, aerobic nitrogen fixation is accomplished by restricting such activity to the anaerobic internal milieu of the heterocyst1. In such species, both differentiation and nif gene expression are repressed by combined nitrogen sources. Finally, some unicellular and filamentous cyanobacterial species have recently been shown to fix nitrogen under aerobic conditions without the benefit of morphologically evident structures, such as heterocyst walls, to protect against oxygen2,3. These species are particularly puzzling because, unlike Azotobacter which can fix nitrogen aerobically by consuming oxygen through vigorous respiration, the cyanobacteria evolve oxygen photosynthetically.