The trans-factors for aldolase B gene

本工作的目的是鉴定（1）醛缩酶B（AldB）基因启动子中的顺式元件，以确定该基因在肝脏中的组织特异性转录;（2）与这些顺式元件结合的反式因子。在过去的两年里，我们已经能够证明有三个这样的顺式元件，A-位点（-98/-123），B部位（-116/-138）和C-位点（-139/-157），以及至少三种反式因子AlF-A、AlF-B和AlF-C，其结合于A-、B-和C-位点，AlF-A似乎是分子量约为70 Kd的蛋白质。AlF-B与HNF-1/LFB-1白蛋白基因启动子区的顺式元件竞争性抑制其与A位点的结合，提示AlF-B与HNP-1/LFB-1白蛋白基因启动子区的顺式元件相似。AlF-B是CAAT结合蛋白之一，因为B位点含有CAAT基序序列。AlF-B的热稳定性、组织特异性和分子量与迄今报道的其他CAAT结合蛋白如CBP、CTF/NF-1、NF-Y和C/EBP不同。AlF-B可能是一种新的CAAT结合蛋白，其CAAT基序上游2个核苷酸的胞苷甲基化（-GC*GCCAAT-）完全抑制了AlF-B与B位点元件的结合。我们的前期研究表明，该位置的低甲基化是AldB基因在体内表达所必需的。这一发现支持了AlF-B与B位点元件的结合对该基因在体内的转录起重要作用.总之，我们已经能够证明AldB基因在肝脏中的组织特异性转录主要受反式因子AlF-A、AlF-G和AlF-C的控制。除了这些反式因子外，其中一个顺式元件B位点元件的甲基化在该基因的组织特异性转录中具有另一个重要作用。因此，AldB基因的组织特异性转录受到双重调节。

The aim of this work was to identify (1) the cis-elements in the promoter of aldolase B (AldB) gene for the tissue-specific transcription of this gene in the liver, and (2) the trans-factors which bind to these cis-elements. For the last two years, we have been able to show that there are three such cis-elements, A-site (-98/-123), B-site (-116/-138), and C-site (-139/-157), within the promoter of AldB gene, and at least three trans-factors, AlF-A, AlF-B, and AlF-C, which bind to A-, B-, and C-sites, respectively.AlF-A seems to be the protein with the molecular weight of about 70 Kd. Its binding to A-site is competitively inhibited by the cis-element in the promoter of albumin gene for HNF-1/LFB-1, thus suggesting that this factor is the one similar, if not identical, to HNP-1/LFB-1.AlF-B is one of the CAAT-binding proteins, since the B-site contains CAAT motif sequence. The heat-stability, tissue-specificity, and molecular weight of AlF-B do not resemble those of other CAAT-binding proteins so far reported, such as CBP, CTF/NF-1, NF-Y, and C/EBP. Therefore, it is probable that AlF-B is a novel CAAT-binding protein.The binding of this factor to the B-site element is inhibited completely with the cytidine methylation in vitro at the position two nucleotide upstream of the CAAT motif (-GC*GCCAAT-). Our previous study showed that hypomethylation at this position is essential for the expression of AldB gene in vivo. This finding supports that AlF-B binding to the B-site element id important for the transcription of this gene in vivo.In conclusion, we have been able to show that the tissue-specific transcription of AldB gene in the liver is controlled primarily by trans-factors, AlF-A, AlF-G, and AlF-C. Besides these trans-factors, the methylation of one of the cis-elements, the B-site element, has another important role in the tissue-specific transcription of this gene. Thus, the tissue-specific transcription of AldB gene is doubly regulated.