Transcriptional regulation of meiosis in budding yeast

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y the 26S proteasome (Guttmann-Raviv and Kassir, 2002). In the absence of Ime1, Sok2 represses the transcription of IME1, leading to a decrease in IME1 mRNA. It is not known whether the negative feedback regulation of Ime2 on the transcription of IME1 is mediated only through its effect on Ime1. Since the transcription of IME2 depends on Ime1 (Mitchell et al., 1990; Yoshida et al., 1990) and the half-life of Ime2 is very short (Bolte et al., 2002; Guttmann- Raviv and Kassir, 2002), there are sufficient levels of Ime1 to relieve repression by Sin3 (Washburn and Esposito, 2001) and to activate transcription of EMG. In the absence of Ime1 lack of these two functions leads to shutting down the transcription of EMG. The transcription of MMG and LMG depends on Ime2 (Chu et al., 1998; Chu and Herskowitz, 1998; Friesen et al., 1997; Hepworth et al., 1998; Kihara et al., 1991; Ozsarac et al., 1997; Smith and Mitchell, 1989). In cells depleted for Ime1, Ime2 is absent, and the transcription of the early late and late genes is closed. VIII. Concluding remark. The choice between developmental pathways S. cerevisiae is a simple eukaryote with limited developmental options. Diploid cells may chose between growth in yeast or pseudohyphal forms, and meiosis. Two crucial mechanisms regarding developmental decisions are required for faithful growth, entry into a specific developmental pathway should take place only in response to specific signals, and should be a signal to block entry into an alternative pathway. Concomitant entry into two developmental pathways could lead to chaos. In S. cerevisiae, the MATa and MATα gene products as well as the pachytene surveillance mechanisms ensure that only diploid cells can initiate meiosis. This is a crucial decision, because meiosis in haploid cells would lead to the formation of uneuploid gametes. In S. cerevisiae, the meiotic cycle ends with the formation of dormant spores resistant to hazardous conditions. The decision to exit the mitotic cell cycle depends on the availability of nutrients. Entry into the meiotic cycle in the presence of nutrients can be a “waste”. Several redundant mechanisms ensure that in the presence of glucose the meiotic pathway will be repressed: i. The transcription of IME1 is repressed in the presence of glucose due to the presence of distinct negative elements (i.e. UASru, IREu, UCS1), each responding to a different signal pathway. Thus, if one signal pathway is malfunctioning, the activity of the additional signal pathways will ensure repression. Moreover, several UAS elements (i.e. UASru, IREu, UASrm), each regulated in a distinct manner, activates transcription in the absence of glucose. ii. The activity of Ime1 is repressed in the presence of glucose by two distinct mechanisms. a. Phosphorylation of Ime1 by the Cln/Cdc28 kinase sequesters the protein from the nucleus, and b. 41
Functional interaction between Ime1 and Ume6 that promote transcriptional activation is inhibited in the presence of glucose and nitrogen. The presence of nitrogen also prevents translation of IME1 mRNA and entry of Ime1 into the nucleus. The presence of glucose also inhibits the function of Ime2. Normally Ime2 is available only under meiotic conditions, but cells developed mechanisms to ensure that in the presence of nutrients accidental expression of Ime2 will not lead to entry and completion of meiosis. In the presence of glucose activated Gpa2 binds to and inhibits the function of Ime2. Furthermore, Ime2 is a non-stabile protein, thus under growth conditions the low level of Ime2 does not suffice for initiation of meiosis in the absence of Ime1. Yeast cells use the same regulators, but in reverse directions, to control alternative developmental pathways. Ime2 is a positive regulator of meiosis that prevents pseudohyphae development in growth media with acetate as the sole carbon source (Donzeau and Bandlow, 1999). The activity of the cAMP/PKA signal pathway is a major player in determining the developmental choice yeast cells make. Entry into mitosis with either the yeast form or filamentous morphology requires high activity of this pathway [for reviews see (Gancedo, 2001)], whereas entry into meiosis requires low PKA activity (Matsumoto et al., 1983). Sok2 is a positive regulator of mitosis (Ward et al., 1995) and a negative regulator for the transcription of IME1, meiosis, and filamentous growth (Shenhar and Kassir, 2001; Ward et al., 1995). The negative role of Sok2 in pseudohyphae formation is not clear since its C. albicans homolog, Efg1, that is a negative regulator of meiosis when expressed in S. cerevisiae [complementing sok2∆ (Shenhar and Kassir, 2001)], is a positive regulator of filamentous growth in both S. cerevisiae and C. albicans (Stoldt et al., 1997). Msn2/4 exhibit reverse tasks, it functions as a negative regulator of the mitotic cell cycle (Smith et al., 1998) and filamentous growth (Stanhill et al., 1999) and as a positive regulator for the transcription of IME1 and meiosis (Shenhar and Kassir, 2001). The activity of Sok2 and Msn2/4 in growing cells requires their phosphorylation by PKA (Gorner et al., 1998; Shenhar and Kassir, 2001; Smith et al., 1998; Ward et al., 1995). The use of the same regulators, but in reverse directions for different developmental pathways, ensures that one pathway will be an alternative to the other one. When cells enter mitosis, meiosis will be repressed, whereas when cells enter meiosis, mitosis will be blocked. Thus, a single signal transduction pathway, the cAMP/PKA, is sufficient to control two alternative developmental pathways. Acknowledgment. We thank M. Foiani for his hospitality while writing this review, for fruitful discussions and critical reading of the review. 42
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 and 40: VI. Transcriptional regulation of l
Page 41: The transcriptional activator that
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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