George C X, Samuel C E
Department of Molecular, Cellular, and Developmental Biology, University of California, Santa Barbara, CA 93106, USA.
Proc Natl Acad Sci U S A. 1999 Apr 13;96(8):4621-6. doi: 10.1073/pnas.96.8.4621.
RNA-specific adenosine deaminase (ADAR1) catalyzes the deamination of adenosine to inosine in viral and cellular RNAs. Two size forms of the ADAR1 editing enzyme are known, an IFN-inducible approximately 150-kDa protein and a constitutively expressed N-terminally truncated approximately 110-kDa protein. We have now identified alternative exon 1 structures of human ADAR1 transcripts that initiate from unique promoters, one constitutively expressed and the other IFN inducible. Cloning and sequence analyses of 5'-rapid amplification of cDNA ends (RACE) cDNAs from human placenta established a linkage between exon 2 of ADAR1 and two alternative exon 1 structures, designated herein as exon 1A and exon 1B. Analysis of RNA isolated from untreated and IFN-treated human amnion cells demonstrated that exon 1B-exon 2 transcripts were synthesized in the absence of IFN and were not significantly altered in amount by IFN treatment. By contrast, exon 1A-exon 2 transcripts were IFN inducible. Transient transfection analysis with reporter constructs led to the identification of two functional promoters, designated PC and PI. Exon 1B transcripts were initiated from the PC promoter whose activity in transient transfection reporter assays was not increased by IFN treatment. The 107-nt exon 1B mapped 14.5 kb upstream of exon 2. The 201-nt exon 1A that mapped 5.4 kb upstream of exon 2 was initiated from the interferon-inducible PI promoter. These results suggest that two promoters, one IFN inducible and the other not, initiate transcription of the ADAR1 gene, and that alternative splicing of unique exon 1 structures to a common exon 2 junction generates RNA transcripts with the deduced coding capacity for either the constitutively expressed approximately 110-kDa ADAR1 protein (exon 1B) or the interferon-induced approximately 150-kDa ADAR1 protein (exon 1A).
RNA特异性腺苷脱氨酶（ADAR1）催化病毒和细胞RNA中的腺苷脱氨生成肌苷。已知ADAR1编辑酶有两种大小形式，一种是干扰素诱导的约150 kDa蛋白，另一种是组成型表达的N端截短的约110 kDa蛋白。我们现已鉴定出人类ADAR1转录本的可变外显子1结构，它们起始于独特的启动子，一个是组成型表达的，另一个是干扰素诱导型的。对来自人胎盘的cDNA末端5'-快速扩增（RACE）cDNA进行克隆和序列分析，确定了ADAR1外显子2与两个可变外显子1结构之间的联系，本文将其指定为外显子1A和外显子1B。对从未经处理和经干扰素处理的人羊膜细胞中分离的RNA进行分析表明，外显子1B-外显子2转录本在无干扰素的情况下合成，并且经干扰素处理后其数量没有显著变化。相比之下，外显子1A-外显子2转录本是干扰素诱导型的。用报告基因构建体进行瞬时转染分析，鉴定出两个功能性启动子，分别称为PC和PI。外显子1B转录本起始于PC启动子，其在瞬时转染报告基因测定中的活性不会因干扰素处理而增加。107个核苷酸的外显子1B位于外显子2上游14.5 kb处。位于外显子2上游5.4 kb处的201个核苷酸的外显子1A起始于干扰素诱导型PI启动子。这些结果表明，一个干扰素诱导型和另一个非干扰素诱导型的两个启动子启动ADAR1基因的转录，并且独特外显子1结构与共同外显子2连接处的可变剪接产生了具有推导编码能力的RNA转录本，可编码组成型表达的约110 kDa ADAR1蛋白（外显子1B）或干扰素诱导的约150 kDa ADAR1蛋白（外显子1A）。
