PATHWAYS OF AMMONIUM ASSIMILATION IN THE SOIL ALGA STICHOCOCCUS BACILLARIS NAEG.

Abstract

SUMMARYGlutamine 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 Kf or 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 Kj^ 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.

Nitrogen metabolism in the ectomycorrhizal fungus Laccaria bicolor (R. Mre.) Orton

“…The aminating activities of NADPH-GDH and NADH-GDH were determined at pH 76 by the spectrophotometric procedures described by Ahmad & Hellebust (1986). Alanine aminotransferase activity was determined by coupling with NADH-lactate dehydrogenase (LDH).…”

The Relationship between Inorganic Nitrogen Metabolism and Proline Accumulation in Osmoregulatory Responses of Two Euryhaline Microalgae

“…NH4Cl or NaNO3 as the nitrogen source and nutrient enrichments as described previously (17). In MSX studies, media containing 0.4 and 2 mm MSX were prepared for S. bacillaris and C. autotrophica, respectively, according to Ahmad and Hellebust (3,4). Cell densities and growth constants (,u) were determined as described previously (4).…”

Partial characterization of enzymes of nitrogen metabolism in Chlorella autotrophica Shihira & Krauss

“…There have been reports of a similar inducible NADPH-GDH in several microorganisms (Tischner & Lorenzen, 1980;Everest & Syrett, 1983;Martin, Msatef & Botton, 1983;Ahmad & Hellebust, 19866;Bascomb & Schmidt, 1987;Jennings, 1989;Ahmad et al, 1990;Schwartz, Kusnan & Fock, 1991). Interestingly, in the green alga Stichococcus bacillaris (Everest & Syrett, 1983 ;Ahmad & Hellebust, 1986ft) NADPH-GDH exhibits a normal rate response curve for ammonium concentration with a single Kv alue of 1-2 mM, whereas in another green alga, Chlorella sorokiniana (Tischner, 1984;Bascomb & Schmidt, 1987), The NADPH-GDH exhibits dual kinetics with respect to ammonium concentration, showing a low K^^ of < 5 mM and a high K,,, of > 20 mM.…”

“…The GDH Stichococcus bacillaris (Everest & Syrett, 1983; pathway leads to the direct synthesis of glutamate by Ahmad & Hellebust, 19866), GDHs from various reductive amination of 2-oxoglutarate, whereas in plant and algal sources exhibit high K^^ values the GS/GOGAT cycle, ammonium is first incor-(10-80 mM) for ammonium (Stewart et al, 1980; porated by GS as amide nitrogen into glutamine Ahmad & Hellebust, 19846). In contrast, the Kŵ hich then in the second step, catalyzed by values for ammonium of GS from various sources GOGAT, is transferred to the a-amino position of 2-are in the micromolar range (Stewart & Rhodes, oxoglutarate to form glutamate.…”