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. lute a. The predominant activity was that of cytosolic GS 1; the relative p roportion of plastidic GS 2 activity changed, however, depending on th e 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 effect ed 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 tetrame ric states. Both oligomers of GS 2 and octameric GS 1 were active unde r all growth conditions applied whereas tetrameric GS 1 was not active when the roots were grown under light-dark changes with NO3- as the m ajor N-source. The molecular masses of the subunits were identical for both isoenzymes. Glutamine synthetase 1 was composed of up to four di fferent 38-kDa subunits and two different 41-kDa subunits; GS 2 was as sembled 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 su bunits. The subunit composition of GS 1, but not of GS 2, changed depe nding on the growth conditions of the roots. Changes in GS 1 subunit c omposition were correlated with changes in GS 1 activity. The differen t growth conditions induced the specific assembly of different GS 1 is oenzymes which could, however, not be separated by anion-exchange chro matography but became evident only after two-dimensional sodium dodecy l sulfate-polyacrylamide gel electrophoresis.