Breckenridge D G, Watanabe Y, Greenwood S J, Gray M W, Schnare M N
Program in Evolutionary Biology, Canadian Institute for Advanced Research, Department of Biochemistry, Dalhousie University, Halifax, Nova Scotia B3H 4H7, Canada.
Proc Natl Acad Sci U S A. 1999 Feb 2;96(3):852-6. doi: 10.1073/pnas.96.3.852.
In the flagellated protozoon Euglena gracilis, characterized nuclear genes harbor atypical introns that usually are flanked by short repeats, adopt complex secondary structures in pre-mRNA, and do not obey the GT-AG rule of conventional cis-spliced introns. In the nuclear fibrillarin gene of E. gracilis, we have identified three spliceosomal-type introns that have GT-AG consensus borders. Furthermore, we have isolated a small RNA from E. gracilis and propose, on the basis of primary and secondary structure comparisons, that it is a homolog of U1 small nuclear RNA, an essential component of the cis-spliceosome in higher eukaryotes. Conserved sequences at the 5' splice sites of the fibrillarin introns can potentially base pair with Euglena U1 small nuclear RNA. Our observations demonstrate that spliceosomal GT-AG cis-splicing occurs in Euglena, in addition to the nonconventional cis-splicing and spliced leader trans-splicing previously recognized in this early diverging unicellular eukaryote.
在有鞭毛的原生动物纤细裸藻中，其特征性核基因含有非典型内含子，这些内含子通常两侧为短重复序列，在mRNA前体中形成复杂的二级结构，并且不遵循传统顺式剪接内含子的GT-AG规则。在纤细裸藻的核纤维蛋白基因中，我们鉴定出了三个具有GT-AG共有边界的剪接体类型内含子。此外，我们从纤细裸藻中分离出一种小RNA，并基于一级和二级结构比较提出，它是U1小核RNA的同源物，U1小核RNA是高等真核生物顺式剪接体的重要组成部分。纤维蛋白内含子5'剪接位点的保守序列可能与裸藻U1小核RNA形成碱基对。我们的观察结果表明，除了此前在这种早期分化的单细胞真核生物中所认识到的非常规顺式剪接和剪接前导序列反式剪接外，剪接体GT-AG顺式剪接也在裸藻中发生。
