Molecular Studies on a Novel Receptor-type Transcription factor, AhR/Arnt.

以2，3，7，8-四氯二苯并二恶英为代表的环境污染物进入细胞后，除了诱导药物代谢酶外，还产生多种不良生物学效应，如致畸、促肿瘤、免疫功能下降和表皮发育不良。这些作用被认为是由Ah受体（AhR）介导的。我们克隆了AhR的cDNA，并通过序列分析推测其一级结构，结果表明AhR是一种受体型转录因子，其N端具有新的bHLH和PAS结构域。PAS是果蝇Per、Arnt（Ah受体核转运蛋白）和果蝇Sim中的保守序列，免疫化学分析表明，AhR和Arnt形成复合物，与诱导型增强子XRE序列结合。AhR/Arnt复合物在体外与Sp1协同作用促进CYP 1A 1基因的转录，模拟了DNA转移实验中该基因的转录。这两种转录因子在其同源DNA元件上相互作用以增强转录。从小鼠胚胎cDNA文库中克隆了B/HLH/PAS家族新成员的cDNA克隆，并通过RNA印迹杂交和整体原位杂交研究了它们的表达。它们在发育中的胚胎的有限组织中特异性表达，提示它们在发育中的功能作用。其中一个克隆是mSim，这是一个人类基因的小鼠对应物，被认为是唐氏综合症的致病基因。

When taken up in the cells, environmental pollutants usually represented by 2,3,7,8 tetrachlorodibenzodioxin exert various adverse biologicl effects such as teratogenesis, tumour promotion, immuno-decifiency, and epidermal dysplasia, in addition to induction of drug-metabolizing enzymes. These effects are considered to be mediated by Ah receptor (AhR). we cloned cDNA for AhR and deduced its primary structure from sequence analysis, which turned out to be a receptor-type transcription factor with novel bHLH and PAS domains at its N-terminus. The PAS is designated as a conserved sequence among Drosophila Per, Arnt (Ah receptor nuclear translocator) and Drosophila Sim.Immunochemical analysis revealed that AhR andarnt form a complex to bind the XRE sequence, an inducible enhancer sequence to in response to ducers. The AhR/Arnt complex enhanced in vitro transcription of CYP1A1 gene in cooperation with Sp1, mimicking the transcription of the gene in the DNA transfer experiments. The two transcription factors interacted with each other on their cognate DNA elements to enhance the transcription. cDNA clones of novel members of the b/HLH/PAS family were isolated from cDNA libraries of mouse embryos and their expressions were investigated by the RNA blot hybridization and whole mount in situ hybridization. They were expressed specifically in the limited tissues of the developing embryos, suggesting the functional roles in the development. One of the clones is mSim, a mouse counterpart of the human gene which is suggested to be a causative gene for Down Syndrome.