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help--interp. of SUP4 effect

Theresa Camilla Swayne tcs6 at merhaba.cc.columbia.edu
Fri Jul 15 14:10:19 EST 1994

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

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