Effect of glutamine on glutamine-synthetase regulation in the green alga Monoraphidium braunii |
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Authors: | José Manuel Garcia-Fernández Antonio López-Ruiz José Alhama José Manuel Roldán Jesús Diez Dapena |
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Institution: | (1) Departamento de Bioquímica y Biología Molecular, Instituto de Biología Básica y Aplicada, Facultad de Veterinaria, Universidad de Córdoba, Avda. Medina Azahara, s/n, E-14071 Córdoba, Spain |
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Abstract: | Glutamine-synthetase (GS; EC 6.3.1.2) activity and protein levels were measured in crude extracts from Monoraphidium braunii Näegeli, strain 202-7d, cultures grown under different nitrogen sources. Only ammonium and l-glutamine promoted a partial enzyme inactivation, which, in the case of l-glutamine, was accompanied by a significant repression of GS. Methionine sulfoximine (MSX), a strong inhibitor of GS, produced a drastic inactivation of GS which was concomitant with a marked increase in GS protein as measured by rocket immunoelectrophoresis. Such an increase was prevented in the presence of cycloheximide. The effect of the l-glutamine analog on GS activity and protein was partially inhibited if l-glutamine was also added to cell cultures, possibly indicating competition in the transport of these two substances. In addition, the effects of MSX were reversed after longer times when cultures were treated with smaller concentrations of inhibitor. Treatment of cell cultures with azaserine, a specific inhibitor of glutamate synthase, the second enzyme acting in the ammonium assimilation pathway, promoted a strong GS inactivation and a partial repression of this enzyme, which paralleled a specific increase in the intracellular pools of glutamine High-performance liquid chromatography measurements of intracellular amino-acid concentrations showed that glutamine levels correlated negatively with GS concentration. A role for glutamine as a negative effector of GS synthesis is proposed.Abbreviations GS
l-glutamine synthetase
- GOGAT
l-glu-tamine:2-oxoglutarate amidotransferase
- MSX
methionine sulfoximine
During the course of this work, J.A. was supported by a fellowship from Junta de Andalucía, and J.M. G-F. by a fellowship from the Spanish Ministerio de Educatión y Ciencia. This work was supported by the Junta de Andalucía. |
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Keywords: | Glutamine synthesis Glutamine synthetase regulation Methionine sulfoximine Monoraphidium |
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