Pak Julia, Segall Jacqueline
Department of Molecular and Medical Genetics, University of Toronto, Ontario, Canada M5S 1A8.
Mol Cell Biol. 2002 Sep;22(18):6417-29. doi: 10.1128/MCB.22.18.6417-6429.2002.
The NDT80 gene of Saccharomyces cerevisiae, which encodes a global activator of transcription of middle sporulation-specific genes, is first expressed after the activation of early meiotic genes but prior to activation of middle sporulation-specific genes. Both upstream repression sequence 1 (URS1) and mid-sporulation element (MSE) sites are present in the promoter region of the NDT80 gene; these elements have been shown previously to contribute to the regulation of expression of early and middle sporulation-specific genes, respectively, by mediating repression in growing cells and activation at specific times during sporulation. In this study, we have shown that the overlapping windows of URS1- and MSE-mediated repression and activation are responsible for the distinctive premiddle expression pattern of the NDT80 gene. Our data suggest that a Sum1-associated repression complex bound at the NDT80 MSE sites prevents Ime1 tethered at the NDT80 URS1 sites from activating transcription of the NDT80 gene at the time that Ime1-dependent activation of early URS1-regulated meiotic genes is occurring. We propose that a decrease in the efficiency of Sum1-mediated repression as cells progress through the early events of the sporulation program allows the previously inactive Ime1 tethered at the URS1(NDT80) sites to promote a low level of expression of the NDT80 gene. This initial phase of URS1-dependent NDT80 expression is followed by Ndt80-dependent upregulation of its own expression, which requires the MSE(NDT80) sites and occurs concomitantly with Ndt80-dependent activation of a set of middle MSE-regulated sporulation-specific genes. Mutation of IME2 prevents expression of NDT80 in sporulating cells. We show in this study that NDT80 is expressed and that middle genes are activated in cells of an Deltaime2/Deltaime2 Deltasum1/Deltasum1 strain in sporulation medium. This suggests that Ime2 activates expression of NDT80 by eliminating Sum1-mediated repression.
酿酒酵母的NDT80基因编码中期孢子形成特异性基因转录的全局激活因子，它在早期减数分裂基因激活后、中期孢子形成特异性基因激活前首次表达。NDT80基因的启动子区域存在上游抑制序列1（URS1）和中期孢子形成元件（MSE）位点；先前已表明，这些元件分别通过介导生长细胞中的抑制作用和孢子形成过程中特定时间的激活作用，参与早期和中期孢子形成特异性基因表达的调控。在本研究中，我们表明URS1和MSE介导的抑制和激活的重叠窗口导致了NDT80基因独特的前中期表达模式。我们的数据表明，结合在NDT80 MSE位点的Sum1相关抑制复合物可防止拴系在NDT80 URS1位点的Ime1在早期URS1调控的减数分裂基因发生Ime1依赖性激活时激活NDT80基因的转录。我们提出，随着细胞经历孢子形成程序的早期事件，Sum1介导的抑制效率降低，使得先前无活性的拴系在URS1（NDT80）位点的Ime1促进NDT80基因的低水平表达。URS1依赖性NDT80表达的这一初始阶段之后是Ndt80依赖性的自身表达上调，这需要MSE（NDT80）位点，并与一组中期MSE调控的孢子形成特异性基因的Ndt80依赖性激活同时发生。IME2突变会阻止孢子形成细胞中NDT80的表达。我们在本研究中表明，在孢子形成培养基中，NDT80在Deltaime2/Deltaime2 Deltasum1/Deltasum1菌株的细胞中表达，且中期基因被激活。这表明Ime2通过消除Sum1介导的抑制作用来激活NDT80的表达。
