Fekete Erzsébet, Pénzes Fruzsina, Ág Norbert, Scazzocchio Claudio, Flipphi Michel, Karaffa Levente
Department of Biochemical Engineering, Faculty of Science, University of Debrecen, 4032 Debrecen, Hungary.
Juhász-Nagy Pál Doctoral School of Biology and Environmental Sciences, University of Debrecen, 4032 Debrecen, Hungary.
J Fungi (Basel). 2021 Aug 29;7(9):710. doi: 10.3390/jof7090710.
Spliceosomal introns are pervasive in eukaryotes. Intron gains and losses have occurred throughout evolution, but the origin of new introns is unclear. Stwintrons are complex intervening sequences where one of the sequence elements (5'-donor, lariat branch point element or 3'-acceptor) necessary for excision of a U2 intron (external intron) is itself interrupted by a second (internal) U2 intron. In Hypoxylaceae, a family of endophytic fungi, we uncovered scores of donor-disrupted stwintrons with striking sequence similarity among themselves and also with canonical introns. Intron-exon structure comparisons suggest that these stwintrons have proliferated within diverging taxa but also give rise to proliferating canonical introns in some genomes. The proliferated (stw)introns have integrated seamlessly at novel gene positions. The recently proliferated (stw)introns appear to originate from a conserved ancestral stwintron characterised by terminal inverted repeats (45-55 nucleotides), a highly symmetrical structure that may allow the formation of a double-stranded intron RNA molecule. No short tandem duplications flank the putatively inserted intervening sequences, which excludes a DNA transposition-based mechanism of proliferation. It is tempting to suggest that this highly symmetrical structure may have a role in intron proliferation by (an)other mechanism(s).
剪接体内含子在真核生物中普遍存在。内含子的获得和丢失在整个进化过程中都有发生，但新内含子的起源尚不清楚。双内含子是复杂的居间序列，其中U2内含子（外部内含子）切除所需的序列元件之一（5'-供体、套索分支点元件或3'-受体）本身被第二个（内部）U2内含子打断。在内生真菌的一个科——炭角菌科中，我们发现了大量供体中断的双内含子，它们彼此之间以及与典型内含子都有显著的序列相似性。内含子-外显子结构比较表明，这些双内含子在不同的分类群中已经增殖，但在一些基因组中也产生了不断增殖的典型内含子。增殖的（双）内含子已无缝整合到新的基因位置。最近增殖的（双）内含子似乎起源于一个保守的祖先双内含子，其特征是末端反向重复序列（45-55个核苷酸），这是一种高度对称的结构，可能允许形成双链内含子RNA分子。假定插入的居间序列两侧没有短串联重复，这排除了基于DNA转座的增殖机制。很诱人的一种推测是，这种高度对称的结构可能通过其他机制在内含子增殖中发挥作用。
