Iwaki A, Iwaki T, Goldman J E, Liem R K
Department of Pathology, Columbia University College of Physicians and Surgeons, New York, New York 10032.
J Biol Chem. 1990 Dec 25;265(36):22197-203.
Two major classes of mRNAs for the alpha-crystallin B chain (or alpha(B)crystallin), about 0.9 and 1.2 kilobases in length, are expressed in rat brain. To examine the structures of these mRNAs, we isolated cDNA clones from rat brain and genomic DNA from rat liver. Characterization of these clones as well as Northern blot analysis indicated that the various mRNAs differed in the lengths of their 5' leader sequences. RNase protection assays revealed that the gene for alpha-crystallin B chain contains multiple start sites. The transcriptional start sites of the longer mRNAs are preceded by a putative CAAT box and that of the shorter mRNA by a putative TATA box. The shorter mRNA encodes the alpha-crystallin B chain protein, whereas the longer mRNA contained three extra small open reading frames upstream of the AUG start codon for the protein. The shorter mRNA is abundant in lens, heart, muscle, and kidney, while the longer mRNAs are constitutively expressed at low levels in a wide variety of tissues. The shorter mRNA was increased by treatment with phorbol 12-myristate 13-acetate in rat C6 glioma cells. Since there is only a single copy of the alpha-crystallin B chain gene, our results indicate that the two classes of mRNAs are generated by alternative transcriptional initiation from different promoters and their expressions are regulated differentially.
大鼠脑中表达两种主要类型的α-晶状体蛋白B链（或αB-晶状体蛋白）mRNA，长度分别约为0.9和1.2千碱基。为了研究这些mRNA的结构，我们从大鼠脑中分离出cDNA克隆，并从大鼠肝脏中分离出基因组DNA。对这些克隆的表征以及Northern印迹分析表明，各种mRNA的5'前导序列长度不同。核糖核酸酶保护试验显示，α-晶状体蛋白B链基因包含多个起始位点。较长mRNA的转录起始位点之前有一个假定的CAAT框，较短mRNA的转录起始位点之前有一个假定的TATA框。较短的mRNA编码α-晶状体蛋白B链蛋白，而较长的mRNA在该蛋白的AUG起始密码子上游包含三个额外的小开放阅读框。较短的mRNA在晶状体、心脏、肌肉和肾脏中含量丰富，而较长的mRNA在多种组织中以低水平组成性表达。在大鼠C6胶质瘤细胞中，用佛波醇12-肉豆蔻酸酯13-乙酸酯处理可增加较短mRNA的表达。由于α-晶状体蛋白B链基因只有一个拷贝，我们的结果表明，这两类mRNA是由不同启动子的选择性转录起始产生的，并且它们的表达受到不同的调节。
