The Molecular Biology of Cyanobacteria

Volume 1 of the series Advances in Photosynthesis pp 487-517

Assimilatory Nitrogen Metabolism and Its Regulation

  • Enrique FloresAffiliated withInstituto de Bioquímica Vegetal y Fotosíntesis, Universidad de Sevilla-CSIC, Facultad de Biología
  • , Antonia HerreroAffiliated withInstituto de Bioquímica Vegetal y Fotosíntesis, Universidad de Sevilla-CSIC, Facultad de Biología

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The element nitrogen (N) constitutes about 5–10% of the dry weight of a cyanobacterial cell. The purpose of this chapter is to review the assimilatory pathways which in free-living cyanobacteria lead from different extracellular N-sources to cellular N-containing components. Inorganic nitrogen in the form of ammonium is incorporated into glutamine and glutamate via the glutamine synthetase/glutamate synthase cycle. The glnA gene, encoding glutamine synthetase, has been characterized in a number of cyanobacteria. Glutamate (and glutamine) distribute N to other organic compounds by means of transaminases, and glutamate is itself a precursor of some other nitrogenous metabolites. Ammonium can be taken up from the external medium by the cyanobacterial cell, but it can also be derived from other nutrients, essentially N2, nitrate and urea. Many cyanobacteria are able to fix N2 under aerobic conditions. Strategies for protecting nitrogenase from O2 in cyanobacteria include the temporal separation of nitrogenase activity and photosynthetic O2 evolution, and in some filamentous cyanobacteria, the differentiation of heterocysts (cells specialized in N2 fixation). A detailed characterization of nif genes has only been performed in a heterocyst-forming cyanobacterium. Nitrate reduction has been found to use photosynthetically reduced ferredoxin as an electron donor, and genes encoding nitrate transport and reduction proteins have been identified and shown to constitute an operon. Some amino acids like arginine and glutamine can also contribute N to some cyanobacteria; however, urea and amino acid utilization have been poorly investigated thus far. Pathways of N assimilation in cyanobacteria are induced upon ammonium deprivation, ammonium being the preferred N source. A gene, ntcA, encoding a transcriptional regulator required for expression of proteins subjected to nitrogen control has been identified. A major theme for future research is how information about the N status of the cell is sensed and transduced to the protein(s) effecting regulation of gene expression.