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Regulation of the subunit composition of tomato plastidic glutamine synthetase by light and the nitrogen source

Andrea Migge, Gudrun Meya, Elisa Carrayol, Bertrand Hirel and Thomas W. Becker
Planta
Vol. 200, No. 2 (1996), pp. 213-220
Published by: Springer
Stable URL: http://www.jstor.org/stable/23384900
Page Count: 8
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Regulation of the subunit composition of tomato plastidic glutamine synthetase by light and the nitrogen source
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Abstract

The co-action of light and the N-source in the regulation of the expression of the single-copy gene encoding plastidic glutamine synthetase (GS-2) and of the multigene family encoding cytosolic glutamine synthetase (GS-1) was investigated in the cotyledons of tomato (Lycopersicon esculentum L.). Light, acting at red/far red or at blue regions of the spectrum increased the abundance of the GS-2 gene product and induced a modification of GS-2 subunits, resulting in the appearance of two GS-2 proteins exhibiting different molecular weights. The magnitude of the light stimulation of GS-2 gene expression was independent of the nitrogen source. However, following red- or far-red-light treatment of etiolated tomato cotyledons, two GS-2 proteins were found when nitrate was the N-source, while only one GS-2 protein was present with ammonium as the sole nitrogen source. Thus, light of specific wavelengths and N-substrates seem to act in concert to regulate GS-2 subunit composition. Tomato GS-1 gene expression was unaffected by light. Ammonium provided externally increased the level of the tomato GS-1 protein. Irrespective of the N-source or the light quality, the GS-1 subunits were represented by polypeptides of similar molecular weight in tomato cotyledons. However, phosphinothricin-induced inhibition of GS activity resulted in the appearance of at least one additional GS-1 polypeptide in etiolated or in green tomato cotyledons. In addition, impairment of GS activity in green tomato cotyledons by phosphinothricin was correlated with an increased level of the GS-1 transcript. Taken together, our data suggest a metabolic control of GS-1 gene expression in green tomato cotyledons.

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