Doetsch N A, Thompson M D, Hallick R B
Department of Molecular and Cellular Biology, University of Arizona, Tucson 85721, USA.
Mol Biol Evol. 1998 Jan;15(1):76-86. doi: 10.1093/oxfordjournals.molbev.a025850.
The fourth intron of the Euglena gracilis chloroplast photosystem II gene, psbCi4, is a 1,605-bp twintron composed of two group III introns and a coding locus for a 458-aa polypeptide, mat1, located in the internal intron. psbCi4 homologs have been identified in seven euglenoids, including E. myxocylindracea, E. viridis, E. deses, E. pisciformis, Cryptoglena pigra, Eutreptia sp., and Lepocinclis beutschlii. All of the species examined contain both the group III twintron and the mat1 locus, revealing a more widespread occurrence of group III introns than previously known. The L. beutschlii mat1 locus is interrupted by two novel mini-group II introns of 224 and 258 nt, the smallest group II introns yet identified. Reverse transcriptase polymerase chain reaction analysis confirmed the splicing boundaries of the external and internal E. myxocylindracea, E. viridis, and E. deses introns as well as the novel L. beutschlii mat1 introns. As determined by comparative phylogenetic analysis, group III introns contain a structural homolog of group II intron domain VI. The mat1 loci encode peptide motifs characteristic of group II intron maturases. A group III intron-encoded protein whose predicted sequence is similar to group II intron-encoded maturases and a bona fide domain VI within group III introns are compelling evidence for a common ancestor of group II and group III introns.
纤细裸藻叶绿体光系统II基因psbCi4的第四个内含子是一个1605碱基对的双内含子，由两个III类内含子和一个位于内部内含子中的458个氨基酸多肽mat1的编码位点组成。已在七种裸藻中鉴定出psbCi4同源物，包括粘圆柱裸藻、绿色裸藻、柔弱裸藻、鱼形裸藻、隐球藻、真裸藻属和贝氏鳞孔藻。所有检测的物种都同时含有III类双内含子和mat1位点，这表明III类内含子的分布比以前所知的更为广泛。贝氏鳞孔藻的mat1位点被两个新的224和258个核苷酸的小型II类内含子打断，这是迄今鉴定出的最小的II类内含子。逆转录酶聚合酶链反应分析证实了粘圆柱裸藻、绿色裸藻和柔弱裸藻外部和内部内含子以及贝氏鳞孔藻新的mat1内含子的剪接边界。通过比较系统发育分析确定，III类内含子包含II类内含子结构域VI的结构同源物。mat1位点编码II类内含子成熟酶特有的肽基序。一种III类内含子编码的蛋白质，其预测序列与II类内含子编码的成熟酶相似，以及III类内含子内一个真正的结构域VI，是II类和III类内含子有共同祖先的有力证据。
