Transcriptional regulation of meiosis in budding yeast

Transcriptional activation of the mid late genes depends on three positive regulators, Ndt80
and Ime2 whose effects are most probably mediated through the MSE like sequence (Friesen et
al., 1997), and Rim1 (Rim101) through an unidentified region (Bogengruber et al., 1998). The
effect of Rim1 may be indirect (see IIIC3). The repression activity of NRE DIT depends on Ssn6,
Tup1, Rox3, Sin4 and Spe3 (Friesen et al., 1997; Friesen et al., 1998). Rox3 and Sin4 are
subunits of the mediator complex of RNA polymerase II (Gustafsson et al., 1997; Myer and
Young, 1998) that repress transcription of many genes (Hanna-Rose and Hansen, 1996). SPE3
encodes Spermidine synthase (Hamasaki-Katagiri et al., 1997), and accordingly, addition of
spermidine to the media leads to increased repression (Friesen et al., 1998). The way by which
these complexes are specifically recruited to NRE DIT is not known.
XBP1 is a mid-late gene whose transcription is initiated at the same time as that of DIT1,2, but
unlike these genes its transcription is non-transient (Mai and Breeden, 2000). Accordingly, its
regulated transcription is not mediated via a NRE DIT element (Mai and Breeden, 2000). XBP1
encodes a DNA-binding protein that functions as a repressor when fused to lexA. Deletion of
XBP1 leads to a delay and reduction in the percentage of asci, while meiotic divisions are
properly controlled (Mai and Breeden, 2000). It is not known how this effect of Xbp1 is
mediated.
C. Late genes
The transcription of the 5 late meiotic genes peaks following completion of meiotic divisions at
8-12 hours in sporulation media (Law and Segall, 1988). A representative of these genes is
SPS100 that is involved in spore wall formation (Law and Segall, 1988). The transcription of
SPS100 depends on Ime1, Ndt80, and Ama1 (Cooper et al., 2000; Hepworth et al., 1998). AMA1
encodes a meiosis-specific subunit of APC/C that is required for degradation of CLB1 in the
meiotic cycle (Cooper et al., 2000). The transcription of AMA1 is induced at the same time as
MMG, but since it does not carry the Ndt80 biding site (Chu et al., 1998) its mode of regulation is
not known. In addition, meiosis-specific splicing by the EMG Mer1 (Engebrecht and Roeder,
1990) determines the accumulation of Ama1 protein (Cooper et al., 2000). Diploid cells deleted
for AMA1 arrest with a short spindle prior to the first meiotic division (Cooper et al., 2000).
Genetic analysis suggests that Ama1 is dispensable for the second meiotic division, but is
required for spore formation (Cooper et al., 2000). Further work is required to determine if Ama1
is directly involved with transcriptional regulation of late genes, for example by degradation of a
transcriptional repressor, or if the effect on transcription is mediated by a checkpoint mechanism.
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