LIGHT-INDUCED SYNCHRONOUS CULTURES, AN EXCELLENT TOOL TO STUDY THE CELL-CYCLE OF UNICELLULAR GREEN-ALGAE

Light-induced synchronous cultures of unicellular green algae represen t an excellent tool for the study of cell cycle progression and the me chanisms of cell cycle regulation. Key points of the cell cycle have b een characterized and homologues of yeast proteins playing a role in c ell cycle control have been identified in algae. Cell cycle progressio n is governed by a timer setting a circadian clock and by a sizer dete rmining the number of divisions. Synchronous cultures are moreover ide ally suited to investigate cell cycle related metabolic events, e.g. D NA, RNA and protein synthesis. Special attention has been given to pho tosynthesis which changes remarkably during the cell cycle. Besides fu nction and composition of the photosynthetic apparatus the regulation of chloroplast protein synthesis is a main subject of recent studies w ith synchronous algae. Among the various properties of the cells that are changing periodically during the cell cycle only few are directly related to cell cycle progression. Others such as variations in protei n synthesis are caused by the change of light and dark periods. In add ition to the change in environment a circadian clock entrained by the light/dark-rhythm may influence metabolic processes in synchronous cel ls, e.g. photosynthesis and expression of certain genes coding for chl oroplast proteins. Recent results on the molecular mechanism of the cl ock as studied with synchronous cultures are summarized. Certainly the mechanism of synchronization by light and dark phases is still not cl arified unequivocally but it seems clear that photosynthetic activity is a prerequisite for synchronous growth.