Abstract
Nitrate is a major nitrogen source of cyanobacteria, which is actively transported into the cell. While most fresh-water strains of cyanobacteria have the ABC-type nitrate/nitrite transporter (ABC-NRT), Nostoc punctiforme has the NNP (Nitrate-Nitrite Porter) family nitrate/nitrite transporter NrtP, which is homologous to NRT2 of plants. Unlike ABC-NRT having high affinity (apparent Km≈1 μM) for both nitrate and nitrite, NrtP showed lower affinity for nitrite (apparent Km≈10 μM) than for nitrate (apparent Km≈1μM). NrtP was distinct from ABC-NRT also in its insensitivity to ammonium. Nitrate assimilation by the N. punctiforme cells was nevertheless inhibited by ammonium because of inhibition of nitrate reductase. These results implied that when N. punctiforme takes up ammonium from medium, it stops nitrate reduction but continues uptake and accumulation of nitrate. The physiological significance of these traits is discussed in terms of adaptation to oligotrophic environments.
