Abstract
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 G 1 phase in the light, and remaining at a high level during DNA synthesis in the dark.
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