The cyanobacteria are prokaryotes that belong to the domain of Bacteria and are characterized by their capability to perform oxygenic photosynthesis. The eubacterial character of cyanobacteria is reflected in the structure of their transcriptional apparatus which is similar to that of well-characterized eubacteria like Escherichia coli (Houmard 1994) and in their ability to act as recipients of DNA in conjugation with E. coli promoted by wide host-range plasmids like RP-4 (Wolk et al. 1984). The dominant mode of growth of cyanobacteria is photoautotrophy, fixing CO2 through the reductive pentose phosphate pathway (Stanier, Cohen-Bazire 1977). Other metabolic points contributing to CO2 fixation in cyanobacteria are those of carbamoyl phosphate synthetase and phosphoenolpyruvate carboxylase. Because the cyanobacteria lack 2-oxoglutarate dehydrogenase, they have an incomplete Krebs cycle that essentially functions anabolically rendering 2-oxoglutarate, which is used for the incorporation of N into the cellular organic materials. The main route for N incorporation in cyanobacteria is the glutamine synthetase-glutamate synthase pathway, although some cyanobacteria also express a glutamate dehydrogenase activity (Flores, Herrero 1994). The inorganic nitrogenous compound that is incorporated into C skeletons is ammonium, which can be taken up from the extracellular medium or produced intracellularly from other N sources.

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