Transcriptional regulation of meiosis in budding yeast

y the 26S proteasome (Guttmann-Raviv and Kassir, 2002). In the absence of Ime1, Sok2
represses the transcription of IME1, leading to a decrease in IME1 mRNA. It is not known
whether the negative feedback regulation of Ime2 on the transcription of IME1 is mediated only
through its effect on Ime1. Since the transcription of IME2 depends on Ime1 (Mitchell et al.,
1990; Yoshida et al., 1990) and the half-life of Ime2 is very short (Bolte et al., 2002; Guttmann-
Raviv and Kassir, 2002), there are sufficient levels of Ime1 to relieve repression by Sin3
(Washburn and Esposito, 2001) and to activate transcription of EMG. In the absence of Ime1 lack
of these two functions leads to shutting down the transcription of EMG. The transcription of
MMG and LMG depends on Ime2 (Chu et al., 1998; Chu and Herskowitz, 1998; Friesen et al.,
1997; Hepworth et al., 1998; Kihara et al., 1991; Ozsarac et al., 1997; Smith and Mitchell, 1989).
In cells depleted for Ime1, Ime2 is absent, and the transcription of the early late and late genes is
closed.
VIII. Concluding remark. The choice between developmental pathways
S. cerevisiae is a simple eukaryote with limited developmental options. Diploid cells may chose
between growth in yeast or pseudohyphal forms, and meiosis. Two crucial mechanisms regarding
developmental decisions are required for faithful growth, entry into a specific developmental
pathway should take place only in response to specific signals, and should be a signal to block
entry into an alternative pathway. Concomitant entry into two developmental pathways could lead
to chaos. In S. cerevisiae, the MATa and MATα gene products as well as the pachytene
surveillance mechanisms ensure that only diploid cells can initiate meiosis. This is a crucial
decision, because meiosis in haploid cells would lead to the formation of uneuploid gametes.
In S. cerevisiae, the meiotic cycle ends with the formation of dormant spores resistant to
hazardous conditions. The decision to exit the mitotic cell cycle depends on the availability of
nutrients. Entry into the meiotic cycle in the presence of nutrients can be a “waste”. Several
redundant mechanisms ensure that in the presence of glucose the meiotic pathway will be
repressed: i. The transcription of IME1 is repressed in the presence of glucose due to the presence
of distinct negative elements (i.e. UASru, IREu, UCS1), each responding to a different signal
pathway. Thus, if one signal pathway is malfunctioning, the activity of the additional signal
pathways will ensure repression. Moreover, several UAS elements (i.e. UASru, IREu, UASrm),
each regulated in a distinct manner, activates transcription in the absence of glucose. ii. The
activity of Ime1 is repressed in the presence of glucose by two distinct mechanisms. a.
Phosphorylation of Ime1 by the Cln/Cdc28 kinase sequesters the protein from the nucleus, and b.
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