Replicative senescence in Schizosaccharomyces pombe

Unlike Saccharomyces cerevisiae which divides by budding, Schizosaccharomyces pombe (S. pombe) divides by binary fission and hence named as fission yeast. They grow by elongation at their ends and divide by medial fission to produce two daughter cells and in this sense, it undergo morphologically symmetric division (however, the division is not symmetric in all aspects).

Minois et al. (2006) studied the replicative senescence of S. pombe by measuring the occurrence and intensity of protein carbonylation (a marker of senescence) in single and symmetrically dividing cells of S. pombe. In S. cerevisiae, cabonylated proteins are segregated unevenly between mother and daughter cells during cytokinesis, with most of the carbonylated proteins retained by the mother cell. However, in S. pombe, proteins are not segregated to one half of the cell but are shared between the two cells. They found that carbonylated proteins are never segregated to mother cell alone but are shared between the two. However, with increase in carbonylation, they are shared less equally. This pattern of sharing is opposite to that of S. cerevisiae. This indicates that in S. pombe, damaged cells give rise to damaged cells itself. In a population, most of the cells have only low levels of carbonylation whereas a few of the cells show high levels of carbonylation which is consistent with senescence model of S. cerevisiae in which a gradual accumulation of carbonylated proteins occurs as the cells undergo aging. Cells with high levels of carbonylation exhibit lower fitness and are less likely to divide. However, since majority of the cells have only limited carbonylated proteins, this indicate that the pool of young cells are large enough to prevent the extinction of the population.

Thus, Minois et al. (2006) have shown that cells of S. pombe also undergo aging. However, unlike S. cerevisiae, the carbonylated proteins are shared between the mother and the daughter cells and hence the old cells of S. pombe generate old cells itself.

My model of E.coli senescence is almost similar to that of S. pombe. This will be discussed later.

Next- Replicative senescence in Caulobacter crescentus

Minois et al. (2006). Symmetrically dividing cells of the fission yeast Schizosaccharomyces pombe do age. Biogerontology 7:261-267.