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PATHWAYS OF AMMONIUM ASSIMILATION IN THE SOIL ALGA STICHOCOCCUS BACILLARIS NAEG.
Author(s) -
AHMAD IFTIKHAR,
HELLEBUST JOHAN A.
Publication year - 1986
Publication title -
new phytologist
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 3.742
H-Index - 244
eISSN - 1469-8137
pISSN - 0028-646X
DOI - 10.1111/j.1469-8137.1986.tb00596.x
Subject(s) - ammonium , glutamine synthetase , glutamate synthase , glutamate dehydrogenase , nitrogen assimilation , biochemistry , biology , heterotroph , enzyme , chemistry , glutamine , glutamate receptor , bacteria , amino acid , organic chemistry , receptor , genetics
SUMMARY Glutamine synthetase (GS) and NADPH‐glutamate dehydrogenase (GDH) are key enzymes for ammonium assimilation in Stichococcus bacillaris Naeg. (UTEX 314). Cells growing with different concentrations of nitrate and ammonium contain activities of both GS and NADPH‐GDH in excess of required nitrogen assimilation rates under autotrophic as well as heterotrophic conditions. The activity of GS increases 2 to 3 fold, and NADPH‐GDH 10 to 15 fold when the concentration of ammonium in the medium is lowered from 2 to 0–2 mM, or when nitrate is present as the nitrogen source. The activity of ferredoxin‐dependent glutamate synthase (GOGAT) is similar to, or high than that of GS. In this alga. NADPH‐GDH has a low K m for ammonium (1–6 mM), and is capable of functioning effectively over the pH range occurring in the chloroplast stroma. A possible role of NADPH‐GDH in nitrogen assimilation by S. bacillaris is supported by the observation that the alga grows normally in the presence of methionine sulphoximine (MSX), when high levels of this enzyme are present, while GS is almost completely inhibited. Stichococcus bacillaris also contains high activities of NADH‐GDH, but the K m for ammonium of this GDH (33 mM) appears too high for it to function effectively in nitrogen assimilation. The activity of NADH‐GDH increases 3 to 5 fold when cells growing photoautotrophically are transferred to heterotrophic conditions, suggesting a catabolic role for this enzyme.