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Nitrogen fixation in photosynthetic bacteria and in cyanobacteria is characterized by several features of nif gene organization and regulation not encountered in the well-studied diazotrophs Klebsiella pneumoniae and Azotobacter species. The filamentous cyanobacteria, in addition, differentiate specialized cells for nitrogen fixation, the heterocysts, and this cellular process requires the coordinated expression of many genes, up to one-fifth of the 7-Mb genome of Anabaena species. Significant progress has been made, in recent years, in the elucidation of these novel features in two model organisms, the purple non-sulfur bacterium Rhodobacter capsulatus and the cyanobacterium Anabaena. In order to appreciate these novelties, it will be useful to recall the essential elements of nif gene organization and regulation in Klebsiella and Azotobacter. The key regulatory genes are ntrB, ntrC, nifA and nifL. All of the genes encoding nitrogenase components, FeMoCo synthesis factors, assembly factors and electron transfer proteins are transcribed from promoters recognized by σ54-RNA polymerase, a form of holoenzyme that requires activation by the enhancer-binding proteins NtrC or NifA. In the case of most nif genes, the enhancer binds NifA. The exception is nifA itself, whose enhancer binds NtrC. In these bacteria, oxygen represses nif gene expression (indirectly) by oxidizing the redox protein NifL which, in oxidized form, prevents the binding of NifA to its enhancer sequences. Ammonia represses at two levels: by converting NifL to a NifA-binding form and by preventing transcription of the nifA genethrough the action of NtrB and NtrC. Details of the role of NifL are described in the article by R. A. Dixon elsewhere in this volume. NtrB and NtrC are an environment-sensing pair, in which NtrB is the sensor. When the level of fixed nitrogen falls, it phosphorylates NtrC. The phosphorylated form of NtrC binds to its enhancer sequences and its ATPase activates σ54-RNA polymerase. |
