Tae H J, Luo X, Kim K H
Department of Biochemistry, Purdue University, West Lafayette, Indiana 47907.
J Biol Chem. 1994 Apr 8;269(14):10475-84.
The gene for acetyl-CoA carboxylase, the rate-limiting enzyme in the biosynthesis of long-chain fatty acids, contains two distinct promoter regions, denoted PI and PII, which control the generation of different forms of mRNA. Multiple forms of acetyl-CoA carboxylase (ACC) mRNA with 5'-end heterogeneity are generated as a result of differential splicing of two primary transcripts formed under the control of these two promoters. PI is responsible for the generation of class I mRNAs of ACC, which are induced in a tissue-specific manner under lipogenic conditions. PII generates class II mRNAs of ACC, which are expressed constitutively. Possible mechanisms for the regulation of PI under normal physiological conditions and agents that activate the promoter have been investigated. PI contains a TATA and a CCAAT box. In addition to these sequences, this promoter contains a 28-CA repeat sequence 220 bases upstream from the transcription initiation site; the presence of this sequence leads to about 70% repression of the basal promoter activity. Repression by the 28-CA repeat sequence requires the GCAAT sequence in the CCAAT box. The negative effect of the 28-CA repeat sequence is relieved by a CCAAT/enhancer-binding protein (C/EBP), which binds to the GCAAT sequence. Insertion of the 28-CA repeat sequence into the thymidine kinase promoter results in repression that can also be relieved by the C/EBP gene product. However, the same sequence exerts no effect on ACC promoter II, which has no CCAAT box. During the differentiation of 30A5 preadipocytes into adipocytes, the expression of class I ACC mRNA and C/EBP mRNA is coordinately increased. Therefore, the presence of the CA repeat in the promoter may be responsible for the inactivity of PI, and C/EBP may be one of the factors that is responsible for the activation of PI under lipogenic conditions. Interaction of the CA repeat and the CCAAT box in the repression and derepression of the ACC gene provides a novel function for the CCAAT box and C/EBP in gene regulation.
乙酰辅酶A羧化酶是长链脂肪酸生物合成中的限速酶，其基因包含两个不同的启动子区域，分别称为PⅠ和PⅡ，它们控制着不同形式mRNA的产生。由于在这两个启动子控制下形成的两种初级转录本的差异剪接，产生了具有5′端异质性的多种形式的乙酰辅酶A羧化酶（ACC）mRNA。PⅠ负责ACCⅠ类mRNA的产生，这类mRNA在生脂条件下以组织特异性方式被诱导。PⅡ产生ACCⅡ类mRNA，其组成性表达。已经研究了正常生理条件下PⅠ的调控机制以及激活该启动子的因子。PⅠ含有一个TATA盒和一个CCAAT盒。除了这些序列外，该启动子在转录起始位点上游220个碱基处含有一个28个CA的重复序列；该序列的存在导致基础启动子活性约70%的抑制。28个CA重复序列的抑制作用需要CCAAT盒中的GCAAT序列。CCAAT/增强子结合蛋白（C/EBP）与GCAAT序列结合后，可解除28个CA重复序列的负效应。将28个CA重复序列插入胸苷激酶启动子中会导致抑制作用，C/EBP基因产物也可解除这种抑制。然而，相同序列对没有CCAAT盒的ACC启动子Ⅱ没有影响。在30A5前脂肪细胞分化为脂肪细胞的过程中，Ⅰ类ACC mRNA和C/EBP mRNA的表达协同增加。因此，启动子中CA重复序列的存在可能导致PⅠ无活性，而C/EBP可能是生脂条件下激活PⅠ的因素之一。ACC基因抑制和去抑制过程中CA重复序列与CCAAT盒的相互作用为CCAAT盒和C/EBP在基因调控中提供了一种新功能。
