Glutamine synthetase isoenzymes, oligomers and subunits from hairy roots of Beta vulgaris L. var. lutea

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 NO3- as the major N-source. Growth with NH4+ 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 NO3- 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.