Glutamine synthetase isoenzymes, oligomers and subunits from hairy roots of Beta vulgaris L. var. lutea |
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Authors: | Gisela Mäck |
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Institution: | Albrecht-von-Haller-Institut für Pflanzenwissenschaften, Universit?t G?ttingen, Untere Karspüle 2, D-37073 G?ttingen, Germany, DE
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Abstract: | A cytosolic and a plastidic isoenzyme of glutamine synthetase (GS; EC 6.3.1.2) were separated from hairy roots of Beta vulgaris L. var. lutea. The predominant activity was that of cytosolic GS 1; the relative proportion of plastidic GS 2 activity changed, however,
depending on the growth conditions. Maximum activity of both isoenzymes was measured after growth with NO−
3 as the major N-source. Growth with NH+
4 as the sole N-source or growth in constant darkness resulted in a significant decrease in GS 1 activity, whereas GS 2 activity
was much less effected and thus contributed as much as 25% of total root GS activity. The isoenzymes GS 1 and GS 2 were active
both in the octameric and tetrameric states. Both oligomers of GS 2 and octameric GS 1 were active under all growth conditions
applied whereas tetrameric GS 1 was not active when the roots were grown under light-dark changes with NO−
3 as the major N-source. The molecular masses of the subunits were identical for both isoenzymes. Glutamine synthetase 1 was
composed of up␣to four different 38-kDa subunits and two different 41-kDa subunits; GS 2 was assembled from one type of 38-kDa
subunit and one type of 41-kDa subunit. The GS␣2 subunits were most probably identical to two of the GS␣1 subunits. The subunit
composition of GS 1, but not of GS 2, changed depending on the growth conditions of the roots. Changes in GS 1 subunit composition
were correlated with changes in GS 1 activity. The different growth conditions induced the specific assembly of different
GS 1 isoenzymes which could, however, not be separated by anion-exchange chromatography but became evident only after two-dimensional
sodium dodecyl sulfate-polyacrylamide gel electrophoresis.
Received: 30 May 1997 / Accepted: 28 August 1997 |
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Keywords: | :Beta Glutamine synthetase Hairy roots Isoenzyme (GS oligomers subunits) Nitrogen meta‐ bolism |
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