Transcriptional regulation of meiosis in budding yeast

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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. 39
The transcriptional activator that binds to and activates transcription of the late genes is not known. The late meiosis-specific genes SPS100 and DTT1 show basal levels of expression in vegetative growth conditions (Jung and Levin, 1999). This expression depends on a functional MAPK cascade that transmits the cell wall integrity signal (Jung and Levin, 1999). Activation of the MAPK Slt2/Mpk1, or deletion of Rlm1, the transcription factor whose activity is regulated by it, reduces the expression the late meiosis specific genes (Jung and Levin, 1999). It is not known whether Rlm1 binds the promoter region of late genes, or if it is required for their expression under meiotic conditions. VII. A feedback loop controlling meiosis. The transcription of all meiosis-specific genes is transient, reflecting the need for the function of these genes for a limited period. Nevertheless, it is important to point that transient transcription does not necessarily reflect transient availability of proteins. For instance, in the mitotic cell cycle, the transcription of genes involved in DNA replication, such as CLB5, POL1, or POL12 is periodic, and identically regulated (Lowndes et al., 1991; Schwob and Nasmyth, 1993). But the steady state level of Pol1 and Pol12 is constant (Foiani et al., 1995), whereas the level of Clb5 is periodic (Irniger and Nasmyth, 1997). Periodicity of Clb5 is accomplished by its regulated degradation (Irniger and Nasmyth, 1997). The pattern of protein expression of many meiosisspecific genes is not known, however, the two main positive regulators of meiosis, Ime1 and Ime2 are non-stable proteins whose protein levels mimic the level of their RNA. This suggests that at least in the case of Ime1 and Ime2, the transient availability might be a sign of requirement for only a short period. In agreement with this view, over expression of Ime1 in meiotic cultures leads (in some strains) to inefficient sporulation, a substantial increase in non-disjunction, the formation of 2-spored rather than 4-spored asci, and a reduction in sporulation (Shefer-Vaida et al., 1995; Sherman, 1992). Positive feedback loops control the transcription of both IME1 and IME2 (Fig. 14). Positive autoregulation by Ime1 is required to relieve repression mediated by Sok2, and thus lead to an increase in Ime1 level (Shenhar and Kassir, 2001). Positive autoregulation accelerates the availability of Ime1 and Ime2, and this enhanced expression might be essential for proper entry and progression through the meiotic cycle. The transient transcription of all meiosis-specific genes is explained by the transient availability of Ime1, Ime2, and Ndt80. Phosphorylation of Ime1 by Ime2 leads to its degradation 40
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 and 32: expressed only in the absence of gl
Page 33 and 34: structure of hCDK2 reveals that cyc
Page 35 and 36: terminal non-essential domain (Komi
Page 37 and 38: is dependent on Ime2, but in cells
Page 39: VI. Transcriptional regulation of l
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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