| The aim of this study was to define both the roles of glycogen and trehalose during the yeast life cycle and the regulatory mechanisms that allow these roles to be filled successfully. Wild-type cells growing exponentially on glucose-containing medium have low levels both of glycogen and trehalose. Specific starvation for nitrogen, sulfur, or phosphorous, in the presence of excess glucose, leads to accumulation of both reserves, although the kinetics of glycogen accumulation differ substantially from those of trehalose. If an energy-limited culture grows until glucose is exhausted from the medium, both reserves are accumulated. Trehalose accumulation begins at about the time of glucose exhaustion. Glycogen accumulation begins considerably earlier; strangely, its onset does not seem to correlate with cell density, the concentration of glucose in the medium, or the concentrations of glucose-derived waste products. Cells grown to an energy-limited stationary phase remain viable for over 100 days, during which time they slowly degrade both their glycogen and trehalose. Loss of cell viability is correlated with the depletion of these reserve carbohydrates.;Mutants defective specifically in trehalose accumulation were sought using a replica-printing screening technique. Of the 53,000 clones screened, 219 had reduced trehalose levels, but all had reduced levels of glycogen as well. It is not known why no mutants were obtained; several possible explanations are discussed.;Mutants completely defective in the accumulation of normal or abnormal glycogen show behavior indistinguishable from wild type in a variety of situations, including sporulation, in which glycogen has been thought to be involved, suggesting that glycogen serves no essential role during the yeast life cycle. In contrast, mutants defective in glycogen and trehalose accumulation show defects in a variety of situations in which reserve carbohydrates are involved. |
