Transcriptional regulation of meiosis in budding yeast

media and nitrogen depletion) phosphorylation of Ime1and Ume6 by Rim11 and Rim15 promotes
the interaction of Ime1 with Ume6. Ime1 function is required to relieve repression by Sin3 and to
activate transcription. In addition, transcriptional activation depends on histone acetylation by
Gcn5.
Fig. 12. Ime2 is highly homologous to hCDK2.
A. Sequence alignment of Ime2 to hCDK2. B, Ribbon depiction of the crystal structure of
hCDK2 (Protein Data Bank structure code 1HCL). C. Ribbon depiction of the homology based
model of Ime2 based on the CDK structure. The following stretches of residues are colored for
clarity: the ATP binding site (yellow), The PSTAIRE site (orange), the Catalytic site (red) and the
T-loop (blue). The Ime2 model was prepared using the 3D-PSSM modeling algorithm (Kelley et
al., 2000). The figures were prepared using InsightII (Accelrys).
Fig. 13. Transcriptional regulation of middle meiosis-specific genes.
Ndt80 is the transcriptional activator binding to the MSE element present in the 5’ region of all
middle meiosis-specific genes (MMG). A subset of MMG carries an MSE* site (SMK1 in
comparison to CLB1) that can be bound by Sum1. Sum1 associates with Hst1 that functions as
histone deacetylase, thus leading to silencing under vegetative growth conditions and at early
meiotic times. Under meiotic conditions relief of repression is due to degradation of Sum1 by
Ime2. Sum1 also represses the transcription of NDT80, since it carries both an MSE and MSE*
elements. In addition, NDT80 carries two URS1 elements that are bound by Ume6. In vegetative
growth media Ume6 recruits the Sin3/Rpd3 histone deacetylase complex that represses
transcription. Under meiotic conditions Ume6 recruits Ime1 that can activate transcription, but
only in the absence of Sum1. Ime2 phosphorylates Ndt80, and this phosphorylation may
contribute to the complete transcriptional activity of Ndt80. In the absence of meiotic
recombination the pachytene checkpoint inhibits degradation of Sum1 and phosphorylation of
Ndt80. Consequently, the level of Ndt80 is reduced, and the unphosphorylated Ndt80 is impaired
in activating the transcription of MMG.
Fig. 14. Positive and negative feedback loops control meiosis.
Ime1 shows positive autoregulation that is required to relieve repression mediated by Sok2 and
thus for its high-level transcription. Expression of early meiosis-specific genes (EMG) requires
relief of repression of Sin3 and transcriptional activation, two functions that are promoted by
Ime1. The transcription of middle genes (MMG) depends on Ime2 and Ndt80. Transcriptional
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