Nitrogen fixation by diazotrophic bacteria is a strictly regulated process which is controlled at multiple levels to prevent futile synthesis and activity of nitrogenase and accessory functions. Aerobic diazotrophs face the "oxygen paradox" (Marchal, Vanderleyden 2000) in that they require oxygen to generate ATP via oxidative phosphorylation and, at the same time, oxygen should be kept at a very low level to prevent irreversible damage of nitrogenase. A number of regulatory principles is conserved among many diazotrophs while other control mechanisms are specific to individual species. In rhizobia, a largely common set of regulatory components have been identified which, however, are integrated in disparate regulatory networks with distinct features (for reviews, see Kaminski et al. 1998; Fischer 1994, 1996). Here, a summary is presented about the current knowledge of the regulatory circuits that control symbiotic nitrogen fixation genes in the soybean symbiont Bradyrhizobium japonicum. Special emphasis is given to a global regulatory system (RegSR) whose function is probably not restricted to nitrogen fixation.
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