EFFECTS OF DIFFERENT CARBON FLUXES ON G(1) PHASE DURATION, CYCLIN EXPRESSION, AND RESERVE CARBOHYDRATE-METABOLISM IN SACCHAROMYCES-CEREVISIAE

By controlled addition of galactose to synchronized galactose-limited Saccharomyces cerevisiae cultures, the growth rate could be regulated while external conditions were kept constant. By using this method, th e G(1) phase duration was modulated and expression of cell cycle-regul ated genes was investigated. The expression of the cyclin genes CLN1 a nd CLN2 was always induced just before bud emergence, indicating that this event marks the decision to pass Start. Thus, G(1) phase elongati on was not due to a slower accumulation of the CLN1 and CLN2 mRNA leve ls. Only small differences in CLN3 expression levels were observed. Th e maximal SWI4 expression preceded maximal CLN1 and CLN2 expression un der all conditions, as expected for a transcriptional activator. But w hereas SWI4 was expressed at about 10 to 20 min, before CLN1 and CLN2 expression at high growth rates, this time increased to about 300 min below a particular consumption rate at which the G(1) phase strongly e longated. In the slower-growing cultures, also an increase in SWI6 exp ression was observed in the G(1) phase. The increase in G(1) phase dur ation below a particular consumption rate was accompanied by a strong increase in the reserve carbohydrate levels. These carbohydrates were metabolized again before bud emergence, indicating that below this con sumption rate, a transient increase in ATP flux is required for progre ssion through the cell cycle. Since Start occurred at different cell s izes under different growth conditions, it is not just a certain cell size that triggers passage through Start.