CONTROL BY NUTRIENTS OF GROWTH AND CELL-CYCLE PROGRESSION IN BUDDING YEAST, ANALYZED BY DOUBLE-TAG FLOW-CYTOMETRY

To gain insight on the interrelationships of the cellular environment, the properties of growth, and cell cycle progression, we analyzed the dynamic reactions of individual Saccharomyces cerevisiae cells to cha nges and manipulations of their surroundings. We used a new flow cytom etric approach which allows, in asynchronous growing S. cerevisiae pop ulations, tagging of both the cell age and the cell protein content of cells belonging to the different cell cycle set points. Since the cel l protein content is a good estimation of the cell size, it is possibl e to follow the kinetics of the cell size increase during cell cycle p rogression. The analysis of the findings obtained indicates that both during a nutritional shift-up (from ethanol to glucose) and following the addition of cyclic AMP (cAMP), two important delays are induced. T he preexisting cells that at the moment of the nutritional shift-up we re cycling before the Start phase delay their entrance into S phase, w hile cells that were cycling after Start are delayed in their exit fro m the cycle. The combined effects of the two delays allow the cellular population that preexisted the shift-up to quickly adjust to the new growth condition. The effects of a nutritional shift-down were also de termined.