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 Sequences of five shark species were also analysed for their structure and included in phylogeny because of unpublished work reporting introns in their ITS, obtaining congruent results.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.

中文翻译：

---

真核生物内部转录间隔区中的第I组内含子。

第I组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各样的原生生物和各种原核生物的小亚基（SSU）和大亚基（LSU）中，但是在ITS区域却从未发现它们。在这项研究中，分析了异常长的ITS区域的真菌和紧密相关的单细胞生物（聚多聚菌，线粒体线虫，西方变形虫和单核核仁）。尽管重复插入是其他真核生物中长ITS的原因，但本文分析的序列大小增加似乎是由于ITS-1或ITS-2中存在内含子。可以根据第I组内含子模型将鉴定出的插入片段折叠成二级结构，并在基于保守核心的系统发育中将它们聚集成内含子。另外，对于线虫，变形虫和核子菌，一旦去除间隔子内含子，就可以推断出更常规的ITS-2结构。还对五个鲨鱼物种的序列进行了结构分析，并将其纳入系统发育研究中，因为尚未发表有关在ITS中报告内含子的工作，从而获得了一致的结果。总体而言，本文提供的数据表明间隔区可能包含内含子。