Transcriptional regulation of meiosis in budding yeast

for this domain suppresses ime1∆ in both wt and ids2∆ cells (Sia and Mitchell, 1995). The steadystate
level of Ids2 is decreased and then disappears in cells shifted to meiotic conditions (Sia and
Mitchell, 1995).
RIM4 is an EMG required for high-level expression of EMG, premeiotic DNA replication,
timely and efficient commitment to meiotic recombination, nuclear division, and spore formation
(Deng and Saunders, 2001; Soushko and Mitchell, 2000). Rim4 function is mediated through
both the Ime1 and Ime2 transcriptional activation pathways (Soushko and Mitchell, 2000). rim4∆
diploids over-expressing Ime2 are sporulation proficient, suggesting that Rim4 functions
upstream of Ime2 (Soushko and Mitchell, 2000). Rim4 carries two RNA recognition motifs that
are important for its role in meiosis (Soushko and Mitchell, 2000). The mode by which Rim4
affects transcription is not known, though it was proposed that it might stabilize IME2 mRNA
(Deng and Saunders, 2001; Soushko and Mitchell, 2000).
3. Sin3 and Rpd3. Random screening for mutations preventing expression of MMG
identified NDT80 and SIN3 (Hepworth et al., 1998). Sin3 as well as Rpd3 are required for the
transcription of MMG, suggesting that they function also as positive regulators, or that their
function as positive regulators might be due to repression of a negative regulator (Hepworth et al.,
1998). Diploid cells deleted for SIN3 or RPD3 arrest at the same point as ndt80 cells, namely,
following the completion of premeiotic DNA replication, as mono nucleate cells (Hepworth et al.,
1998). Point of arrest is not due to defects in recombination, since concomitant deletion of SPO13
and SPO11 does not allow a bypass of the arrest, and the triple mutant cells remain sporulation
deficient (Hepworth et al., 1998; Xu et al., 1995).
B. Negative regulators of MMG.
SMK1 is a middle gene subject to silencing by binding of Sum1 to the MSE* element in its
promoter (Xie et al., 1999). Sum1 also represses the transcription of NDT80, in cells deleted for
SUM1 premature expression of NDT80 is observed (Pak and Segall, 2002a). Sum1 associates
with Hst1, and the resulting complex, containing additional proteins, exhibits NAD + dependent
histone deacetylase activity (Pierce et al., 1998; Pijnappel et al., 2001; Rusche and Rine, 2001).
Deletion of either SUM1 or HST1 leads to expression of SMK1 in vegetative growth conditions
(Xie et al., 1999). This expression is probably independent of Ndt80, since NDT80 remains silent
in sum1∆ or hst1∆ cells (Xie et al., 1999). Relief of repression is regulated by Sum1 availability:
The transcription of SUM1 is constitutive, but the level of Sum1 protein is reduced prior to
expression of MMG, and then it is induced again. As described above, the transcription of NDT80
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