Genetic regulation of nitrogen assimilation in photosynthetic bacteria

Features of the genetic regulation of nitrogen metabolism in nitrogen-fixing phototrophic bacteria with the anoxygenic type of photosynthesis are reviewed. This group includes bacteria belonging to the family Rhodospirillaceae. The review is focused on the three representatives of this family, Rhodobacter capsulatus, Rhodobacter sphaeroides, and Rhodospirillum rubrum, which are genetically the best studied. Data on pathways of ammonia and nitrate assimilation and on the mechanisms responsible for the transport of ammonium ions and different amino acids in photosynthetic purple bacteria are presented. Diazotrophic bacteria possess effective regulatory mechanisms ensuring the expression of nitrogen fixation genes only under conditions of limited oxygen and fixed nitrogen. The key role in the regulation of nif gene transcription in R. capsulatus and R. sphaeroides belongs to the NtrB and NtrC proteins, whereas in Rs. rubrum, the transcription of nitrogen fixation genes does not depend on the functioning of these proteins. Apparently, other regulatory systems may be involved in the transcriptional regulation of nif genes in this organism. The characteristic feature of the regulation of nitrogen fixation in R, sphaeroides is the absolute dependence of the glnBA operon expression on the NtrC protein. In addition, the ntrC gene of R. sphaeroides is involved in the regulation of both the nitrogen metabolism and carbon/energy metabolism. Nitrogen fixation in photosynthetic purple bacteria is regulated at the transcriptional level and also at the posttranslational level. In R. capsulatus and Rs. rubrum, the draT and draG gene products are responsible for the regulation of nitrogenase activity through its covalent modification. Nitrogenase activity in R. sphaeroides cells is controlled by another mechanism, since the draT and draG genes were not detected in this bacterium. Moreover, in R. capsulatus, the hvrA gene product responsible for the activation of the photosynthesis apparatus under low-light conditions is involved in the posttranslational regulation of nitrogenase activity. The process of nitrogen fixation in phototrophic purple bacteria is closely associated with the processes of photosynthesis and carbon dioxide fixation. Data showing the interaction between the regulatory systems of these processes are considered.