Ammonium assimilation in bryophytes. L-glutamine synthetase from Sphagnum fallax.

Kahl, Stefan; Gerendás, Jóska; Heeschen, Volker; Ratclifte, R. George; Rudolph, Hansjörg

Cytosolic and plastidic L-glutamine synthetase (EC 6.3.1.2) isoenzymes from Sphagnum fallax Klinggr. (Klinggr. clone 1) were separated by size-exclusion and ion exchange chromatography. The cytosolic enzyme (GS1) was purified to apparent electrophoretic homogeneity. The native enzyme had a molecular mass of 390 ± 20 kDa as estimated by gel filtration and was apparently composed of 8 subunits with molecular masses of 48 kDa. GS1 activity could be measured from pH 6.8 to 8.6 in 50mM imidazole buffer, with a broad optimum between pH 7.2 and 8.0. The Km values were 2.5 mM, 0.5 mM and 0.5 mM for L-glutamate, ammonium and ATP, respectively. The enzyme was inhibited by more than 10 mM ammonium or glutamate. The incorporation of 15NH4 into amino acids was observed in vivo using 15N NMR. Label from ammonium was first detected in the amide N of glutamine, and only subsequently in the amino N of glutamate. Moreover, no assimilation was detected in the presence of the specific GS inhibitor methionine sulfoximine. These observations are consistent with a dominant role for GS in the assimilation of ammonium in Sphagnum.

GLUTAMINE synthetase; LIGASES; PEAT mosses; AMMONIUM; ION exchange chromatography; PLANT physiology

Physiologia Plantarum, 1997, Vol 101, Issue 1, p86

0031-9317

Academic Journal

10.1111/j.1399-3054.1997.tb01823.x