Evidence for the involvement of a cytoplasmic factor in the aging of the yeast Saccharomyces cerevisiae
The life spans of individual Saccharomyces cerevisiae cells were determined microscopically by counting the number of buds produced by each cell to provide a measure of the number of cell generations (age) before death. As the cells aged, their generation times increased five- to sixfold. The generation times of daughter cells were virtually identical to those of their mothers throughout the life spans of the mothers. However, within two to three cell divisions after the daughters were detached from their mothers by micromanipulation, their generation times reverted to that characteristic of their own age. Recovery from the mother cell effect was also observed when the daugthers were left attached to their mothers. The results suggest that senescence, as manifested by the increase in generation time, is a phenotypically dominant feature in yeast cells and that it is determined by a diffusible cytoplasmic factor(s) that undergoes turnover. This factor(s) appeared to be transmitted by a cell not only to its daugther, but also indirectly to its granddaugther. In separate studies, it was determined that the induced deposition of chitin, the major component of the bud scar, in the yeast cell wall had no appreciable effect on life span. We raise the possibility that the cytoplasmic factor(s) that appears to mediate the "senescent phenotype" is a major determinant of yeast life span. This factor(s) may be the product of age-specific gene expression.
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