The cyanobacteria have Amt proteins that can mediate the uptake of ammonium (as well as of its structural analog methylammonium) when it is present at very low concentrations in the extracellular medium. We refer to these proteins as Amtl (Montesinos et al. 1998). Analysis of sequenced genomes has shown however that some cyanobacteria posses additional amt genes, like the amt2 and amt3 genes of the unicellular strain Synechocystis sp. PCC 6803 whose function remains unknown. Nonetheless, mutation of the three amt genes of strain PCC 6803 renders a strain that can still incorporate some ammonium from the extracellular medium (J. Paz-Yepes, unpublished), probably through a mechanism which involves diffusion of unprotonated ammonia followed by trapping of ammonium by glutamine synthetase.
Sources of N other than ammonium which are widely used by cyanobacteria include nitrate (and nitrite), urea, and dinitrogen. Nitrate and nitrite are transported into the cell in many cyanobacteria through an ABC-type transporter known as NrtABCD (Flores, Herrero 1994) or, in some other cyanobacteria, through a major facilitator superfamily permease, NrtP (Sakamoto et al. 1999). Nitrate is intracellularly reduced to nitrite by the ferredoxin-dependent nitrate reductase, and nitrite is reduced to ammonium by the ferredoxin-dependent nitrite reductase (Flores, Herrero 1994). A genetic structure commonly (although not universally) found for the nitrate assimilation genes is: nir (nitrite reductase) - gene(s) encoding the permease - narB (structural gene for nitrate reductase). This gene cluster constitutes an operon (Flores, Herrero 1994) with a clear polarity, i. e. transcripts corresponding to the 5' end of the operon are more abundant than those corresponding to the 3' end (Frías et al. 1997).
As is the case for ammonia, urea can readily diffuse through biological membranes. However, in Synechocystis sp. PCC 6803 and in the Infixing strain Anabaena sp. PCC 7120 an ABC-type transporter which exhibits a high affinity for urea (Ks -100 nM) has been identified (A. Valladares, unpublished). This transporter, encoded in Anabaena sp. by the urtABCDE operon, may have an important role in urea assimilation when urea is available at concentrations too low to allow a significant rate of diffusion into the cell. In cyanobacteria, intracellular urea is degraded to ammonium and CO2 by a conventional bacterial-type, Ni2+-containing urease.
Some cyanobacteria can also assimilate N2 while growing phototrophically, and they have developed different strategies to protect the oxygen-sensitive nitrogenase from both atmospheric and photosynthetically generated oxygen. Separation, either in space or in time, of the nitrogenase and photosynthetic activities is commonly observed in cyanobacteria (Fay 1992). In some filamentous cyanobacteria, nitrogenase is confined to heterocysts, differentiated cells that lack oxygen-evolving photosystem II activity and exhibit a metabolism directed to provide an environment adequate for the nitrogenase activity (Wolk et al. 1994). The nitrogenase structural genes, nifHDK, are expressed in the heterocysts of aerobically grown filaments of Anabaena sp. PCC 7120. Heterocysts and vegetative cells exhibit an intense exchange of metabolites in the N2-fixing cyanobacterial filament (Wolk etal. 1994).
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