In its natural habitat, the yeast Saccharomyces cerevisiae
must cope with large fluctuations in the environmental conditions; to do so, it adapts its metabolism to a large variety of external conditions. One such adaptation is to accumulate reserve carbohydrates such as glycogen and trehalose when nutritional conditions deteriorate (12
). Initially believed to act as storage factors, trehalose and glycogen were later implicated in other roles as well.
The observation that trehalose and glycogen accumulate not only upon carbon starvation but also under other stress conditions such as nitrogen or sulfur starvation, heat shock, or osmotic stress (7
) led to the suggestion that they act as stress protectants rather than as reserve carbohydrates. A role in stress protection has been attributed to trehalose in particular (23
), since in vitro experiments showed that trehalose protects enzymes and membranes during dehydration and heat stress (2
) and therefore might act as a stabilizer of cellular structures under stress conditions (2
). Nevertheless, the relationship between trehalose and glycogen accumulation and stress resistance has remained unclear because mutants in the metabolic pathways of these compounds did not exhibit the expected phenotypes (reviewed in reference 14
). It is only recently that Singer and Lindquist (19
) showed that also in vivo trehalose serves as a protectant during heat shock and prevents denaturation and aggregation of proteins upon heat shock.
Another interesting function for trehalose and glycogen came to attention recently: their possible role in cell cycle progression. Studies using synchronized cultures showed that below a particular sugar flux, trehalose and glycogen levels increased during the G1
phase of the cell cycle and were subsequently degraded upon entry into S phase (18
). These results have led to the suggestion that trehalose and glycogen may be required under low sugar supply to temporarily increase the sugar flux in order for the cell to complete a cell division cycle (18
). Interesting in this respect is also the observation of Küenzi and Fiechter (11
), who showed that a simultaneous change in trehalose and glycogen levels and budding index can be induced in carbon-limited continuous cultures by transiently increasing the sugar flux. The finding that Pho85, a cyclin-dependent kinase, can phosphorylate glycogen synthase isoenzyme 2, resulting in the inactivation of this enzyme, implies a direct link between the cell cycle machinery and trehalose and glycogen metabolism (9
). In addition to Pho85, protein kinase A (PKA), which is activated by glucose via the RAS/cyclic AMP pathway, plays an important role in trehalose and glycogen metabolism (20
). However, although this pathway may play an important role in adjusting glycogen and trehalose levels to the external environmental conditions, no cell cycle-dependent changes in PKA activity have been reported.
To further investigate the role of glycogen and trehalose in cell cycle progression at low growth rates under carbon limitation and in the ability to survive starvation, we have made a mutant unable to synthesize these carbohydrates. This was done by deleting the genes GSY1
) and GSY2
), encoding glycogen synthase isoenzymes 1 and 2, and TPS1
), encoding trehalose-6-phosphate synthase. By growing this mutant and the isogenic wild-type strain in continuous cultures under sugar limitation conditions, we studied the effects of trehalose and glycogen deficiency on metabolism, cell cycle progression, and survival rate under well-defined conditions.