Transcriptional regulation of meiosis in budding yeast

terminal non-essential domain (Kominami et al., 1993). This is deduced from the following
observations: i. Over expression of an Ime2 protein truncated for this domain promotes meiosis in
the presence of either nitrogen or glucose (Kominami et al., 1993), and ii. The in vitro-kinase
activity of Ime2 is higher for a truncated Ime2 protein in comparison to the wt protein (Kominami
et al., 1993). Thus, in the presence of nutrients Ime2 is less, or non-active. The nutrient signal is
transmitted to the C-terminal domain through the Gα protein, Gpa2 (Donzeau and Bandlow,
1999). Ime2 specifically associates with the GTP bound Gpa2 (Donzeau and Bandlow, 1999), a
form whose level is increased in the presence of glucose [for review see (Versele et al., 2001)]. In
addition, the association between Gpa2 and Ime2 requires the presence of nitrogen (Donzeau and
Bandlow, 1999). Cells deleted for GPA2 show similar phenotypes to cells over expressing the
truncated Ime2 protein, namely, sporulation in the presence of glucose and nitrogen (Donzeau
and Bandlow, 1999). Gpa2 associates with the glucose receptor, Gpr1, and transmits the glucose
signal to adenylate cyclase [for review see (Pan et al., 2000)], however, the effect of Gpa2 on
Ime2 is independent of PKA (Donzeau and Bandlow, 1999). This is deduced from the
observation that the bcy1-tpk1 w1 mutation that leads to low activity of PKA does not suppress the
reduction in sporulation exhibited by cells expressing the constitutive active Gpa2G132V protein
(Donzeau and Bandlow, 1999). The in vitro kinase activity of Ime2 on histone H1 is decreased by
the addition of recombinant Gpa2γS (Donzeau and Bandlow, 1999), suggesting that Gpa2 inhibits
the kinase activity of Ime2 (Donzeau and Bandlow, 1999). In vitro kinase assays also demonstrate
that Ime2 phosphorylates Gpa2 (Donzeau and Bandlow, 1999), but the role of this
phosphorylation is not known.
Ime2 kinase activity fluctuates in the meiotic cycle, it first peaks concomitantly with
premeiotic DNA replication and then, concomitantly with nuclear division (Benjamin et al.,
2002). The second increase in activity is regulated by Cdc28 (Benjamin et al., 2002).
The activity of Ime2 is also regulated by Ids2 (Sia and Mitchell, 1995). Over expression of
Ime2 in vegetative growth media is toxic (Bolte et al., 2002; Guttmann-Raviv and Kassir, 2002;
Sia and Mitchell, 1995), and this toxicity is relived in cells deleted for IDS2 (Sia and Mitchell,
1995), suggesting that Ids2 is a positive regulator of Ime2. Ids2 is not required for meiosis (Sia
and Mitchell, 1995), however, in its absence over expression of Ime2 (in the SK1 strain) leads to
the transcription of only the early meiosis-specific genes, while middle and late genes remain
silent and spores are not formed (Sia and Mitchell, 1995). This result suggests that the activity of
Ime2 is distinctly regulated at early and middle meiotic times (Sia and Mitchell, 1995), in
agreement with the finding of Benjamin et al., (Benjamin et al., 2002) reported above. The effect
of Ids2 is mediated through the C-terminal domain of Ime2: Over expression of Ime2 truncated
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