Macromolecular syntheses in the cell cycle mutant cdc25 of budding yeast.

A major control point of the cell cycle in Saccharomyces cerevisiae is a G1 event called 'start'. At start a yeast cell integrates external and internal signals and decides to progress toward mitosis or to choose alternative pathways such as sporulation, conjugation etc. cdc25 is a class II temperature-sensitive start mutant that blocks at restrictive temperature in G1 as round unbudded cells. The arrest of the cell cycle appears to be independent of the carbon and nitrogen sources, and the cell wall of cdc25-arrested cells shows changes similar to those found in cells undergoing entry in to the stationary phase. After a shift to 36 degrees C the increase in cell number of cdc25 cultures is gradually inhibited. The nuclear division cycle appears to be inhibited immediately after the shift and the percentage of budded cells decreases, while cytoplasmic growth, monitored either as increase of adsorbance at 450 nm or as protein accumulation, continues for many hours leading to a progressive increase of mean cell volume and mean protein content per cell. The stable RNA accumulation instead is immediately inhibited and this is partially due to a 50% inhibition of ribosomal RNA synthesis, while the rate of synthesis of ds-killer RNA is relatively unaffected. These data suggest that the CDC25 gene product could be a part of a mechanism that leads yeast cells to choose between the progression towards DNA replication and cell division or to enter into the stationary phase. This mechanism appears to turn off both rRNA accumulation and cell-cycle progression and to activate differentiative pathways in response to environmental restriction.