New trends in photobiology: Light-induced synchronous cultures, an excellent tool to study the cell cycle of unicellular green algae

Abstract

Light-induced synchronous cultures of unicellular green algae represent an excellent tool for the study of cell cycle progression and the mechanisms of cell cycle regulation. Key points of the cell cycle have been characterized and homologues of yeast proteins playing a role in cell cycle control have been identified in algae. Cell cycle progression is governed by a timer setting a circadian clock and by a sizer determining the number of divisions. Synchronous cultures are moreover ideally suited to investigate cell cycle related metabolic events, e.g. DNA, RNA and protein synthesis. Special attention has been given to photosynthesis which changes remarkably during the cell cycle. Besides function and composition of the photosynthetic apparatus the regulation of chloroplast protein synthesis is a main subject of recent studies with 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 protein synthesis are caused by the change of light and dark periods. In addition to the change in environment a circadian clock entrained by the light/dark-rhythm may influence metabolic processes in synchronous cells, e.g. photosynthesis and expression of certain genes coding for chloroplast proteins. Recent results on the molecular mechanism of the clock as studied with synchronous cultures are summarized. Certainly the mechanism of synchronization by light and dark phases is still not clarified unequivocally but it seems clear that photosynthetic activity is a prerequisite for synchronous growth.