DIEL PERIODICITY OF GLUTAMINE-SYNTHETASE ACTIVITY DURING THE CELL-CYCLE OF EMILIANIA-HUXLEYI

Light-dark regulation of glutamine synthetase (GS; EC 6.3.1.2) was inv estigated by following the development of activity and the time-course of the cell-cycle of the marine coccolithophorid, Emiliania huxleyi, under a 16-h photoperiod in synchronous cultures. GS activity showed m arked diel periodicity by exhibiting a maximum value during the dark p eriod and a minimum during the light. Our results suggest that regulat ion of GS activity during the light-dark cycle probably does not opera te only through activation/deactivation mechanisms. Cell division took place from the end of the dark to the first hours of the light period . The succession of the cell-cycle events was analysed by flow cytomet ry. Variations in GS activity appeared to match the demands of the cel l for glutamine, increasing during the G(1) phase in the light, and re maining at a high level during DNA synthesis in the dark. (C) Elsevier , Paris.