In the budding yeast
Saccharomyces cerevisiae entry and successful completion of
meiosis depends on two positive regulators Ime1 and Ime2. Ime1 is a
transcriptional activator that is required for the transcription of
IME2, a serine threonine protein kinase. We show that Ime1 is
not detected in vegetative cultures with either glucose or acetate as
the sole carbon source. A transient induction in the steady state
level of Ime1 is observed in meiotic cultures. This transient
expression correlates with the pattern of IME1 mRNA. Pulse
chase and Western analysis reveal that Ime1 is a non-stable
protein. This stability depends on the kinase activity of
Ime2. Haploid cells overexpressing IME2 show a decrease in the
level of Ime1, whereas diploid cells deleted for IME2 show an
increase in the level of Ime1, even in vegetative media. Using a
temperature sensitive mutation in one of the ATPase genes of the
proteosome, RPT2, we demonstrate that Ime1 is degraded by the
26S proteosome. We also show that Ime2 itself is an extremely
non-stable protein, whose expression in vegetative cultures is
toxic. Thus a negative feedback loop ensures that the activity of Ime1
will be restricted to a short window span. This loop is essential for
efficient meiosis.
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