The vast majority of the purple nonsulfur photosynthetic bacteria are diazotrophs, but the details of the complex regulation of the nitrogen fixation process are well understood only for a few species. Here we review what is known of the well-studied Rhodobacter capsulatus, which contains two different nitrogenases, a standard Mo-nitrogenase and an alternative Fe-nitrogenase, and which has overlapping transcriptional control mechanisms with regard to the presence of fixed nitrogen, oxygen, and molybdenum as well as the capability for the post-translational control of both nitrogenases in response to ammonium. R. capsulatus has two PII proteins, GlnB and GlnK, which play key roles in nitrogenase regulation at each of three different levels: activation of transcription of the nif-specific activator NifA, the post-translational control of NifA activity, and the regulation of nitrogenase activity through either ADP-ribosylation of NifH or an ADP-ribosylation-independent pathway. We also review recent work that has led to a detailed characterization of the molybdenum transport and regulatory system in R. capsulatus that ensures activity of the Mo-nitrogenase and repression of the Fe-nitrogenase, down to extremely low levels of molybdenum.
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