Thanks for all the responses. So far, this is what I have found out with
help from the people who have responded so far to my post and from Finch's
"Longetivity, Senescence, and the Genome" 1990.
The life span of S cerevisiae is about 20-30 cell divisions. This number is
smaller than the potential number of 100 which can be calculated by dividing
the total cell surface by the average scar area. During the senescence phase,
the budding cycle lengthens (by five fold) and the mother cell eventually
dies. Jazwinski et al (1993) showed that v-Ha-ras (Harvey murine sarcoma
virus transforming gene) expression extends the life span by nearly two-fold.
In addition, glucose-fed yeast cells' life span is 50% longer than
the ethanol-fed cells, thus implicating increased mitochondrial respiration
with ethanol in the senescence process. It is likely that oxygen radicals
play a important role in yeast senescence. I wonder if anybody has expressed
superoxide dismutase (SOD) in yeast to see if their life span is extended.
Yet, the definition of maximum life span in yeast would make these studies
complicated as a similar study in Drosophila proved this complexity of
defining the maximum life span of an organism.
A side note for those who are interested in the genome changes with aging:
Jazwinski et al (1991) did not find telomere shortening as yeast cells
senesce. Telomere shortening has been correlated with aging humans and other
mammals.
I mentioned autolysis in my original post. I was confusing S cerevisiae with
filamentous fungi which undergo clonal senescence with the autolysis as the
final step. The morphology of senescent yeast mother cells are characterized
by their wrinkled membrane, loss of turgor, and loss of ability to mate.
Cytolysis seems to be frequent in these old mother cells; otherwise the cell
becomes granular in appearance (Muller, 1985). This seems to indicate
that cell death mechanism in yeast less regulated--or cells just become non
functional.
I appreciate your thoughtful comments. I hope that this post will further
engage comments and discussions.
Regards,
Tae Hoon Kim
tkim at reed.edu
Reed College
Portland, OR 97202
