Institute of Molecular Biology, Academia Sinica, Taipei, Taiwan.
Microbiol Spectr. 2014 Dec;2(6). doi: 10.1128/microbiolspec.MDNA3-0012-2014.
Ciliates are champions in programmed genome rearrangements. They carry out extensive restructuring during differentiation to drastically alter the complexity, relative copy number, and arrangement of sequences in the somatic genome. This chapter focuses on the model ciliate Tetrahymena, perhaps the simplest and best-understood ciliate studied. It summarizes past studies on various genome rearrangement processes and describes in detail the remarkable progress made in the past decade on the understanding of DNA deletion and other processes. The process occurs at thousands of specific sites to remove defined DNA segments that comprise roughly one-third of the genome including all transposons. Interestingly, this DNA rearranging process is a special form of RNA interference. It involves the production of double-stranded RNA and small RNA that guides the formation of heterochromatin. A domesticated piggyBac transposase is believed to cut off the marked chromatin, and the retained sequences are joined together through nonhomologous end-joining processes. Many of the proteins and DNA players involved have been analyzed and are described. This link provides possible explanations for the evolution, mechanism, and functional roles of the process. The article also discusses the interactions between parental and progeny somatic nuclei that affect the selection of sequences for deletion, and how the specific deletion boundaries are determined after heterochromatin marking.
纤毛虫是程序性基因组重排的冠军。它们在分化过程中进行广泛的结构重排，从而极大地改变体细胞基因组的复杂性、相对拷贝数和序列排列。本章以模式纤毛虫四膜虫为重点，它可能是研究最为简单和透彻的纤毛虫。它总结了过去关于各种基因组重排过程的研究，并详细描述了过去十年在理解 DNA 缺失和其他过程方面取得的显著进展。该过程发生在数千个特定的位点上，以去除构成基因组约三分之一的特定 DNA 片段，包括所有转座子。有趣的是，这个 DNA 重排过程是 RNA 干扰的一种特殊形式。它涉及双链 RNA 和小 RNA 的产生，这些 RNA 指导异染色质的形成。一种驯化的猪gyBac 转座酶被认为可以切断标记的染色质，而保留的序列通过非同源末端连接过程连接在一起。许多涉及的蛋白质和 DNA 参与者已被分析并进行了描述。该链接提供了对该过程的进化、机制和功能作用的可能解释。本文还讨论了亲代和子代体细胞核之间的相互作用，这些相互作用影响了用于删除的序列的选择，以及异染色质标记后如何确定特定的删除边界。
