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 investigated 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 marked diel periodicity by exhibiting a maximum value during the dark period and a minimum during the light. Our results suggest that regulation of GS activity during the light-dark cycle probably does not operate 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 cytometry. Variations in GS activity appeared to match the demands of the cell for glutamine, increasing during the G1 phase in the light, and remaining at a high level during DNA synthesis in the dark.