Greetings, fellow yeasties --
During research for a paper on bud site selection in
S. cerevisiae, I came across a suppressor effect that confused me.
In Flescher et al. (1993) JCB 122(2):373-386, "Components required
for cytokinesis are important for bud site selection in yeast,"
an ochre allele, cdc10-10, was isolated. This allele perturbed the
budding pattern of haploid cells, and also caused other morphological
In the presence of cdc10-10 *and* the ochre suppressor SUP4 on a
CEN plasmid, the morphological defects were rescued, but the budding
pattern was still abnormal -- and different from the cdc10-10 pattern.
It didn't seem to be an intermediate phenotype, either.
The cdc10-10 cells budded more often distally than wild-type,
whereas the cdc10-10 SUP4 cells budded more laterally (an indicator
of random budding) than either of the other two strains. I could not
find a mention of a SUP4 control.
The authors suggested that the Q133->Y mutation introduced by
SUP4 might affect targeting of the cdc10-10 protein, enough
to cause an increase in random budding but not enough to disrupt
My questions: is there another possible explanation? Might a nonsense
suppressor be pleiotropic? Naively, I would expect it to be, since it
might cause mutations in as many as a third of all polypeptide species in
the cell. So why does it work?
thanks much for any insight you can offer --
Theresa C. Swayne
Columbia University Dept. of Anatomy & Cell Biology
tcs6 at columbia.edu or swayne at reed.edu