Transcriptional regulation of meiosis in budding yeast

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1. Ndt80. NDT80 encodes a transcriptional activator that binds to the MSE and MSE* elements and activates transcription (Chu and Herskowitz, 1998). Cells deleted for NDT80 arrest following the completion of premeiotic DNA replication and meiotic recombination, at pachytene, as mono nucleate cells (Chu and Herskowitz, 1998; Hepworth et al., 1998; Xu et al., 1995). Ectopic expression of Ndt80 in vegetative growth conditions is sufficient to induce the transcription of most MMG (probably the ones carrying the MSE rather than the MSE* element) (Chu et al., 1998; Chu and Herskowitz, 1998), suggesting that under growth conditions, the second positive regulator, Ime2, is non-essential. Ime2 is also non-essential under meiotic conditions. When Ndt80 is expressed from a heterologous, IME2 independent promoter, the MMG, SPS4, is expressed in both IME2 and ime2∆ cells (Pak and Segall, 2002a). Nevertheless, in the latter, expression of SPS4 is both delayed and lower than in the IME2 diploid cells, suggesting that Ime2 may be required for high and efficient activity of Ndt80 (Pak and Segall, 2002a). This hypothesis is supported by the observation that Ime2 phosphorylates Ndt80 (Benjamin et al., 2002). Ndt80 is a phosphoprotein (Benjamin et al., 2002; Tung et al., 2000), whose in vivo extent of phosphorylation depends on IME2 (Benjamin et al., 2002). Furthermore, in vitro kinase assay demonstrates that Ime2 directly phosphorylates Ndt80 (Benjamin et al., 2002). Ndt80 is also required for high levels of transcription of IME1 as well as for its transient expression (Hepworth et al., 1998). This effect might be mediated through Ime2, since Ndt80 regulates the transcription of IME2, through an MSE element present in its promoter (Chu and Herskowitz, 1998). NDT80 is a meiosis-specific gene whose transcription is dependent on Ime1 and Ime2 (Fig. 13) (Chu and Herskowitz, 1998; Hepworth et al., 1998; Pak and Segall, 2002a). This regulated transcription is mediated by two URS1 and two MSE elements (Chu and Herskowitz, 1998; Hepworth et al., 1998; Pak and Segall, 2002a). One MSE element functions either as a repression or activation element under growth and meiotic condition, respectively, whereas the second one functions only as an activation element (Pak and Segall, 2002a; Xie et al., 1999). Furthermore, both URS1 elements contribute to repression of NDT80 in vegetative cultures and for its high level expression under meiotic conditions [(Pak and Segall, 2002a) and Fig. 13]. The presence of the MSE elements delays NDT80 transcription by the Ume6/Ime1 complex in comparison to other EMG that carry only the URS1 element (Chu and Herskowitz, 1998; Hepworth et al., 1998; Mitchell et al., 1990; Pak and Segall, 2002a; Yoshida et al., 1990). 2. Ime2. IME2 encodes a meiosis-specific protein kinase that shows 58.9% similarity and 37% identity to the human cyclin-dependent kinase, CDK2 (Kominami et al., 1993). The crystal 31
structure of hCDK2 reveals that cyclin A binds to two regions: PSTAIRE and the T-loop (Jeffrey et al., 1995). This binding is required to induce a conformational change that promotes activation of the kinase. Ime2 does not carry the PSTAIRE sequence, but it shows 67% similarity and 52.3% identity to the T-loop region of hCDK2 (Fig. 12). This homology raises the possibility that Ime2 might be activated by binding to a cyclin like molecule, and that in the meiotic cycle it might replace Cdc28, the yeast CDK that regulates initiation and progression in the cell cycle (Lew et al., 1997). The following results support the hypothesis that in the meiotic cycle Ime2 might replace Cdc28: i. Similarly to Cdc28, phosphorylation by Ime2 affects stability of Ime1 Cdh1 and Sic1, the latter two proteins are known substrates of Cdc28 in the mitotic cell cycle (Bolte et al., 2002; Dirick et al., 1998; Guttmann-Raviv and Kassir, 2002). ii. Ime2 is absolutely required for premeiotic DNA replication and meiotic recombination in cdc28-4 cells incubated at the non-permissive temperature (Guttmann-Raviv et al., 2001). However, there are no reports on any yeast proteins that bind to the T-loop like sequence in Ime2 and are required for its kinase activity. Moreover, a recombinant Ime2 protein isolated from E. coli is active in phosphorylating histone H1 (Donzeau and Bandlow, 1999), suggesting that unlike Cdc28, Ime2 is active in the absence of additional yeast proteins. However, phosphorylation of its native substrate, Gpa2 (see below) requires the isolation of Ime2 from yeast cells (Donzeau and Bandlow, 1999), suggesting that specificity, for at least for some substrates, might require post-translation modification or the presence of an activator. Ime2 is a positive regulator of meiosis required for multiple functions: the correct timing and high level transcription of early meiosis-specific genes, the transcription of middle and late genes (Mitchell et al., 1990; Yoshida et al., 1990), the correct timing of entry into premeiotic DNA replication meiotic recombination and nuclear division (Benjamin et al., 2002; Foiani et al., 1996), as well as for ascus formation (Benjamin et al., 2002; Foiani et al., 1996; Mitchell et al., 1990; Yoshida et al., 1990). When Ime2 is over expressed, but only in the SK1 background, it bypasses the requirement for Ime1 for the transcription of EMG and sporulation (Mitchell et al., 1990), suggesting an Ime1 independent pathway for activating the transcription of these genes (Mitchell et al., 1990). It is not known how Ime2, that encodes a kinase, replaces a transcriptional activator, nor if under normal conditions this pathway has any contribution to the transcription of these genes. Ime2 functions also as a negative regulator for the following functions: i. Phosphorylation of Ime1 by Ime2 leads to its degradation by the 26S proteasome (Guttmann-Raviv and Kassir, 2002). It is possible that the absence of Ime1 reestablishes repression by Sok2 and Sin3 [see sections IIIC1.1.2, IVB1.2, and (Shenhar and Kassir, 2001; Washburn and Esposito, 2001)], 32
Page 1 and 2: Transcriptional regulation of meios
Page 3 and 4: 3. Proteins required for the associ
Page 5 and 6: I. Introduction The budding yeast S
Page 7 and 8: et al., 2000). On the other hand, i
Page 9 and 10: the UCS4 element upstream of the CY
Page 11 and 12: egulator of Cyr1, CDC25 encodes the
Page 13 and 14: 1998). Furthermore, unlike IREu, IR
Page 15 and 16: In the absence of glucose Sok2 does
Page 17 and 18: autophosphorylation, including on a
Page 19 and 20: hours in SPM, followed by a decline
Page 21 and 22: 4. The SIN3, UME6, and RPD3 genes.
Page 23 and 24: 1993). Nevertheless, when Ime1 is t
Page 25 and 26: However, direct demonstration of ph
Page 27 and 28: 3. Proteins required for the associ
Page 29 and 30: heterologous promoter does not supp
Page 31: expressed only in the absence of gl
Page 35 and 36: terminal non-essential domain (Komi
Page 37 and 38: is dependent on Ime2, but in cells
Page 39 and 40: VI. Transcriptional regulation of l
Page 41 and 42: The transcriptional activator that
Page 43 and 44: Functional interaction between Ime1
Page 45 and 46: Bowdish, K. S., and Mitchell, A. P.
Page 47 and 48: Foiani, M., Nadjar-Boger, E., Capon
Page 49 and 50: Jeffrey, P. D., Russo, A. A., Polya
Page 51 and 52: Lindgren, A., Bungard, D., Pierce,
Page 53 and 54: Pazin, M. J., and Kadonaga, J. T. (
Page 55 and 56: Shenhar, G. (2001). TRANSCRIPTIONAL
Page 57 and 58: Szent-Gyorgyi, C. (1995). A biparti
Page 59 and 60: Yoshida, M., Kawaguchi, H., Sakata,
Page 61 and 62: Fig. 4. The function of positive an
Page 63 and 64: Fig. 9. A schematic structure of IM
Page 65 and 66: activation by Ime2 is due to phosph
Page 67 and 68: MCK1 Protein kinase required for ef
Page 69: UME3 A cyclin C homolog that associ

Transcriptional regulation of meiosis in budding yeast

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