Residual cell division measurements are unreliable as indicators of the timing of events in the Saccharomyces cerevisiae cell cycle.
We report here an analysis of the execution point of the temperature-sensitive Saccharomyces cerevisiae cell cycle mutant, cdc27-47. When a logarithmically growing culture was shifted from standard growth conditions (strain 27.8B growing in YEPD at 25 degrees C) to the restrictive temperature cell division ceased abruptly and reproducibly within one population doubling time, the extent of cell division indicating an execution point early in the cell cycle. Approximately 50% of stationary-phase cells were able to divide when refed with fresh medium at 37 degrees C, showing that the execution point could be passed before 'start'. This makes the sharp cut-off in cell division difficult to explain. This difficulty was compounded by observations of the cell cycle stage at which individual cells acquired the capacity to divide at 37 degrees C. Half the cells that were budded at the time of a temperature shift-up formed three division-blocked cells, and in 11 of these 13 cases, two were descended from the original mother cell and one from the original bud. Thus, mother and daughter cells pass the execution point independently; daughters usually during G1, and mothers usually in the budded phase of the previous cycle. The sharp cut-off in cell division is therefore spurious, and a mechanism is proposed to account for it, which has implications for the interpretation of the execution points of other cdc mutants. In addition, the expression of the cdc27-47 execution point was modified by both genetic and environmental factors, being affected by carbon source, by the petite condition, and by genetic background. This illustrates the difficulties of interpreting execution point data and the dangers of extrapolation of cell cycle parameters between strains and growth conditions.
D002453:Cell Cycle; D002455:Cell Division; D009154:Mutation; D012441:Saccharomyces cerevisiae / Q000166:cytology* / Q000235:genetics; D013696:Temperature; D013997:Time Factors