EFFECT OF GLUTAMINE ON GLUTAMINE-SYNTHETASE REGULATION IN THE GREEN-ALGA MONORAPHIDIUM-BRAUNII

Glutamine-synthetase (GS; EC 6.3.1.2) activity and protein levels were measured in crude extracts from Monoraphidium braunii Naegeli, strain 202-7d, cultures grown under different nitrogen sources. Only ammoniu m and L-glutamine promoted a partial enzyme inactivation, which, in th e 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 increa se 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 inhib ited if L-glutamine was also added to cell cultures, possibly indicati ng 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 c ultures with azaserine, a specific inhibitor of glutamate synthase, th e second enzyme acting in the ammonium assimilation pathway, promoted a strong GS inactivation and a partial repression of this enzyme, whic h paralleled a specific increase in the intracellular pools of glutami ne High-performance liquid chromatography measurements of intracellula r amino-acid concentrations showed that glutamine levels correlated ne gatively with GS concentration. A role for glutamine as a negative eff ector of GS synthesis is proposed.