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Curr Genet. 2020 Apr;66(2):373-384. doi: 10.1007/s00294-019-01027-0. Epub 2019 Aug 28.
Group I introns are mobile genetic elements that interrupt genes encoding proteins and RNAs. In the rRNA operon, introns can insert in the small subunit (SSU) and large subunit (LSU) of a wide variety of protists and various prokaryotes, but they were never found in the ITS region. In this study, unusually long ITS regions of fungi and closely related unicellular organisms (Polychytrium aggregatum, Mitosporidium daphniae, Amoeboaphelidium occidentale and Nuclearia simplex) were analysed. While the insertion of repeats is responsible for long ITS in other eukaryotes, the increased size of the sequences analysed herein seems rather due to the presence of introns in ITS-1 or ITS-2. The identified insertions can be folded in secondary structures according to group I intron models, and they cluster within introns in conserved core-based phylogeny. In addition, for Mitosporidium, Amoeboaphelidium and Nuclearia, more conventional ITS-2 structures can be deduced once spacer introns are removed. Sequences of five shark species were also analysed for their structure and included in phylogeny because of unpublished work reporting introns in their ITS, obtaining congruent results. Overall, the data presented herein indicate that spacer regions may contain introns.
I 类内含子是可移动的遗传元件，可中断编码蛋白质和 RNA 的基因。在 rRNA 操纵子中，内含子可以插入各种原生生物和各种原核生物的小亚基（SSU）和大亚基（ LSU）中，但在 ITS 区从未发现过。在这项研究中，分析了真菌和密切相关的单细胞生物（聚球藻、达飞衣滴虫、西方阿米巴体和简单核形藻）的异常长的 ITS 区。虽然重复的插入是导致其他真核生物中 ITS 过长的原因，但此处分析的序列大小的增加似乎主要是由于 ITS-1 或 ITS-2 中存在内含子。根据 I 类内含子模型，鉴定出的插入可以在二级结构中折叠，并且它们在基于保守核心的系统发育中聚类。此外，对于 Mitosporidium、Amoeboaphelidium 和 Nuclearia，一旦去除间隔内含子，就可以推导出更常规的 ITS-2 结构。由于未发表的工作报告了其 ITS 中的内含子，因此还分析了五种鲨鱼物种的序列及其结构并将其包含在系统发育中，获得了一致的结果。总体而言，本文提供的证据表明，间隔区可能含有内含子。
