Growth of cyanobacteria in the presence of ammonium results in repression of most of the proteins mentioned above involved in the assimilation of sources of N alternative to ammonium or of ammonium itself. Levels of the nitrate/nitrite transporter, nitrate and nitrite reductases, ammonium/methylammonium permease, urea transporter, and glutamine synthetase are significantly lower in cells grown with ammonium than in cells grown with nitrate or incubated in the absence of combined N (Herrero et al. 2001). In the N2-fixing cyanobacteria, the presence of nitrate is additionally required to attain significant levels of the nitrate and nitrite reductases. Interestingly, as has been shown for the ammonium/methylammonium transport activity in Synechocystis sp. PCC 6803 (Montesinos et al. 1998), nitrate can also determine repression when the cells are incubated with a low supply of CO2. This observation suggests that it is a low C/N ratio, rather than simply the presence of an easily assimilable N source, that determines N repression. In N2-fixing cyanobacteria, heterocyst development and nitrogenase synthesis are repressed not only by ammonium but also by other sources of combined N like nitrate or urea. Nonetheless, these N sources have to render ammonium in order to cause repression, and ammonium metabolism through glutamine synthetase is required for the repressive effect to be manifest (Flores, Herrero 1994).
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