Transcriptional regulation of meiosis in budding yeast

expressed only in the absence of glucose. The mode by which Ime1 relieves repression of
Sin3/Rpd3 and how Isw2 does not repress transcription under these conditions are not known.
Relief of repression does not suffice for transcriptional activation, and requires Gcn5, the histone
acetylase, as well as a transcriptional activation protein, Ime1. Ume6 recruits Ime1 to URS1, and
the interaction between these two proteins depends on the absence of glucose and nitrogen. These
nutrient signals are transmitted to both Ime1 and Ume6, by two protein kinases, Rim15 and
Rim11. Since Rim15 is required for complete phosphorylation of Ume6 in SA and SPM, and
since its kinase activity is inhibited in the presence of glucose by PKA, it is assumed that the
glucose signal that inhibits the association of Ime1 with Ume6 is transmitted, at least partially,
through Rim15. Since Rim15 is only partially required for the interaction of Ime1 with Ume6, the
glucose signal must be transmitted through an additional protein. Rim11 phosphorylates both
Ime1 and Ume6, and this phosphorylation is essential for their interaction. It is not known how
nutrients regulate the function of Rim11.
V. Transcriptional regulation of middle meiosis-specific genes (MMG).
The regulated transcription of middle meiosis-specific genes (MMG) depends on the presence of
two positive elements, MSE (Middle Sporulation Element) (gNCRCAAAA/T) and an Abf1
binding site (Chu et al., 1998; Chu and Herskowitz, 1998; Hepworth et al., 1995; Hepworth et al.,
1998; Ozsarac et al., 1995; Ozsarac et al., 1997; Pierce et al., 1998). The MSE elements present in
different MMG are not identical, some, for example that in CLB1, function only as positive
elements, whereas others, for example that of SMK1 or NDT80 (designated inhere as MSE*),
function also as repression elements in vegetative growth conditions and early meiotic times
(Pierce et al., 1998; Xie et al., 1999). Schematic illustration on the regulation of MMG is
illustrated in Fig. 13, and discussed in the text below.
A. Positive regulators of MMG.
Ndt80 and Ime2 are two meiosis-specific positive regulators absolutely required for the
transcription of MMG (Chu et al., 1998; Chu and Herskowitz, 1998; Hepworth et al., 1998;
Ozsarac et al., 1997). The sequential transcription of MMG following transcription of the early
genes is due to the regulated transcription of both NDT80 and IME2 (Chu and Herskowitz, 1998;
Hepworth et al., 1998).
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