Fekete Erzsébet, Pénzes Fruzsina, Ág Norbert, Ág-Rácz Viktória, Sándor Erzsébet, Scazzocchio Claudio, Flipphi Michel, Karaffa Levente
Department of Biochemical Engineering, Faculty of Science, University of Debrecen, H-4032 Debrecen, Hungary.
Juhász-Nagy Pál Doctoral School of Biology and Environmental Sciences, University of Debrecen, H-4032 Debrecen, Hungary.
J Fungi (Basel). 2022 Apr 13;8(4):397. doi: 10.3390/jof8040397.
Introns are usually non-coding sequences interrupting open reading frames in pre-mRNAs [D1,2]. Stwintrons are nested spliceosomal introns, where an internal intron splits a second donor sequence into two consecutive splicing reactions leading to mature mRNA. In sp. CO27-5, 36 highly sequence-similar [D1,2] stwintrons are extant (sister stwintrons). An additional 81 [D1,2] sequence-unrelated stwintrons are described here. Most of them are located at conserved gene positions rooted deep in the Hypoxylaceae. Absence of exonic sequence bias at the exon-stwintron junctions and a very similar phase distribution were noted for both groups. The presence of an underlying sequence symmetry in all 117 stwintrons was striking. This symmetry, more pronounced near the termini of most of the full-length sister stwintrons, may lead to a secondary structure that brings into close proximity the most distal splice sites, the donor of the internal and the acceptor of the external intron. The stwintrons were overwhelmingly excised by consecutive splicing reactions precisely removing the whole intervening sequence, whereas one excision involving the distal splice sites led to a frameshift. Alternative (mis)splicing took place for both sister and uniquely occurring stwintrons. The extraordinary symmetry of the sister stwintrons thus seems dispensable for the infrequent, direct utilisation of the distal splice sites.
内含子通常是打断前体mRNA中开放阅读框的非编码序列[D1,2]。双内含子是嵌套的剪接体内含子，其中一个内部内含子将第二个供体序列分成两个连续的剪接反应，从而产生成熟的mRNA。在sp. CO27 - 5中，现存36个高度序列相似的[D1,2]双内含子（姐妹双内含子）。本文描述了另外81个[D1,2]序列不相关的双内含子。它们中的大多数位于Hypoxylaceae科深处的保守基因位置。两组在exon - 双内含子连接处均未发现外显子序列偏差，且相位分布非常相似。在所有117个双内含子中都存在明显的潜在序列对称性。这种对称性在大多数全长姐妹双内含子的末端附近更为明显，可能导致一种二级结构，使最远端的剪接位点、内部内含子的供体和外部内含子的受体紧密靠近。双内含子绝大多数通过连续的剪接反应被切除，精确地去除了整个间隔序列，而一次涉及远端剪接位点的切除导致了移码。姐妹双内含子和独特出现的双内含子都发生了可变（错误）剪接。因此，姐妹双内含子的非凡对称性似乎对于不常见的远端剪接位点的直接利用并非必不可少。
