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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. [ABSTRACT FROM AUTHOR]