ANALYSIS OF GENE-EXPRESSION IN MOUSE PREIMPLANTATION EMBRYOS DEMONSTRATES THAT THE PRIMARY ROLE OF ENHANCERS IS TO RELIEVE REPRESSION OF PROMOTERS
ANALYSIS OF GENE-EXPRESSION IN MOUSE PREIMPLANTATION EMBRYOS DEMONSTRATES THAT THE PRIMARY ROLE OF ENHANCERS IS TO RELIEVE REPRESSION OF PROMOTERS
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DOI：
10.1002/j.1460-2075.1993.tb05754.x
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发表时间：
1993-03-01
期刊：
影响因子：
11.4
通讯作者：
DEPAMPHILIS, ML
中科院分区：
文献类型：
--
作者：
MAJUMDER, S;MIRANDA, M;DEPAMPHILIS, ML
增强子通常被简单地视为启动子的延伸，缺乏自身的功能。然而，之前对小鼠植入前胚胎的研究表明，1细胞胚胎可以在没有增强子的情况下有效利用增强子响应性启动子，而2细胞胚胎需要增强子才能达到相同的表达水平。这表明增强子缓解了2-细胞胚胎中的抑制，这在1-细胞胚胎中是不存在的。这里呈现的结果首先证明，1-细胞胚胎免除增强子的能力不是由于缺乏特异性激活蛋白。在GAL 4-VP 16在两个胚胎中激活GAL 4依赖性启动子的条件下，GAL 4-VP 16仅在2-细胞胚胎中激活GAL 4依赖性增强子。此外，增强子的作用不是补偿启动子需求的变化，也不是补偿启动子特异性转录因子水平的降低。天然启动子中的接头扫描突变显示，两个胚胎利用相同的启动子元件，不同启动子的比较显示，这些胚胎具有相同的转录能力。此外，滴定实验显示，在1-细胞胚胎中的Sp1活性较低，其中增强子是不稳定的，而不是在2-细胞胚胎中需要增强子。因此，我们提出，增强子的主要功能，首先与小鼠2-细胞胚胎的形成是显而易见的，是为了防止抑制弱启动子，可能通过改变染色质结构。与这一假设相一致的是，丁酸盐，一种改变染色质结构的试剂，刺激2-细胞胚胎中的启动子，而不是1-细胞胚胎。
Enhancers are generally viewed simply as extensions of promoters, lacking a function of their own. However, previous studies of mouse preimplantation embryos revealed that 1-cell embryos can utilize enhancer-responsive promoters efficiently without an enhancer, whereas 2-cell embryos require an enhancer to achieve the same levels of expression. This suggested that enhancers relieved a repression in 2-cell embryos that is absent in 1-cell embryos. Results presented here demonstrate first that the ability of 1-cell embryos to dispense with enhancers does not result from the absence of specific activation proteins. Under conditions where GAL4-VP16 activated a GAL4-dependent promoter in both embryos, GAL4-VP16 activated a GAL4-dependent enhancer only in 2-cell embryos. Moreover, the role of an enhancer is not to compensate for either changes in promoter requirements, or for reduced levels of promoter-specific transcription factors. Linker-scanning mutations in a natural promoter revealed that both embryos utilized the same promoter elements, and comparison of different promoters revealed that these embryos have equivalent transcriptional capacities. In addition, titration experiments revealed less Sp1 activity in 1-cell embryos where enhancers are dispensable than in 2-cell embryos where enhancers are required. Therefore, we propose that the primary function of enhancers, first evident with formation of a mouse 2-cell embryo, is to prevent repression of weak promoters, probably by altering chromatin structure. Consistent with this hypothesis is the fact that butyrate, an agent that alters chromatin structure, stimulated promoters in 2-cell embryos, but not in 1-cell embryos.