Einvik C, Elde M, Johansen S
Department of Molecular Cell Biology, University of Tromsø, Norway.
J Biotechnol. 1998 Sep 17;64(1):63-74. doi: 10.1016/s0168-1656(98)00104-7.
Protists are unicellular eukaryotes which represent a significant fraction of the global biodiversity. The myxomycete Didymium and the schizopyrenid amoeboflagellate Naegleria are distantly related protists. However, we have noted several striking similarities in life cycle, cell morphology, and ribosomal DNA organization between these organisms. Both have multicopy nuclear extrachromosomal ribosomal DNAs. Here the small subunit ribosomal RNA genes are interrupted by an optional group I twintron, a novel category among the group I introns. Group I twintrons are mobile self-splicing introns of 1.3-1.4 kb in size, with a complex organization at the RNA level. A group I twintron consists of two distinct ribozymes (catalytic RNAs) with different functions in RNA processing, and an open reading frame encoding a functional homing endonuclease--all with prospects of application as molecular tools in biotechnology. Updated RNA secondary structure models of group I twintrons, as well as an example of in vitro ribozyme activity, are presented. We suggest that the group I twintrons have been independently established in myxomycetes and schizopyrenid amoeboflagellates by horizontal gene transfer due to a combination of the phagocytotic behavior in natural environments and the extrachromosomal multicopy nature of ribosomal DNA.
原生生物是单细胞真核生物，在全球生物多样性中占很大比例。黏菌双孢菌和裂殖变形虫鞭毛虫纳格里亚菌是亲缘关系较远的原生生物。然而，我们注意到这些生物在生命周期、细胞形态和核糖体DNA组织方面有几个显著的相似之处。它们都有多拷贝的核外染色体核糖体DNA。在这里，小亚基核糖体RNA基因被一个可选的I类双内含子打断，这是I类内含子中的一个新类别。I类双内含子是大小为1.3 - 1.4 kb的可移动自我剪接内含子，在RNA水平上具有复杂的组织结构。一个I类双内含子由两个在RNA加工中具有不同功能的不同核酶（催化RNA）和一个编码功能性归巢内切酶的开放阅读框组成——所有这些都有作为生物技术分子工具应用的前景。本文展示了I类双内含子的更新RNA二级结构模型以及体外核酶活性的一个例子。我们认为，由于自然环境中的吞噬行为和核糖体DNA的染色体外多拷贝性质，I类双内含子通过水平基因转移在黏菌和裂殖变形虫鞭毛虫中独立形成。
