Developmentally programmed genome rearrangement accompanies differentiation of the silent germline micronucleus into the transcriptionally active somatic macronucleus in the ciliated protozoan Tetrahymena thermophila. Internal eliminated sequences (IES) are excised, followed by rejoining of MAC-destined sequences, while fragmentation occurs at conserved chromosome breakage sequences, generating macronuclear chromosomes. Some macronuclear chromosomes, referred to as non-maintained chromosomes (NMC), are lost soon after differentiation. Large NMC contain genes implicated in development-specific roles. One such gene encodes the domesticated piggyBac transposase TPB6, required for heterochromatin-dependent precise excision of IES residing within exons of functionally important genes. These conserved exonic IES determine alternative transcription products in the developing macronucleus; some even contain free-standing genes. Examples of precise loss of some exonic IES in the micronucleus and retention of others in the macronucleus of related species suggest an evolutionary analogy to introns. Our results reveal that germline-limited sequences can encode genes with specific expression patterns and development-related functions, which may be a recurring theme in eukaryotic organisms experiencing programmed genome rearrangement during germline to soma differentiation.

中文翻译：

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为了精确切除四膜虫基因组重排，需要有种系限制的piggyBac转座酶基因

发展程序的基因组重排伴随着沉默的种系微核分化为纤毛虫四膜虫的转录活性体细胞大核。切除内部消除的序列（IES），然后重新加入以MAC为目的地的序列，而片段化发生在保守的染色体断裂序列上，从而产生大核染色体。一些大核染色体，称为非维持染色体（NMC），在分化后很快消失。大型NMC包含与发育特定角色有关的基因。一个这样的基因编码驯化的piggyBac转座酶TPB6，是功能重要基因外显子中IES的异染色质依赖性精确切除所必需的。这些保守的外显子IES确定了正在发育的大核中的替代转录产物。有些甚至包含独立基因。某些外显子IES在微核中的精确损失以及其他物种在相关物种的大核中的保留的例子表明了与内含子的进化类比。我们的结果表明，种系有限的序列可以编码具有特定表达模式和发育相关功能的基因，这可能是真核生物在种系到体细胞分化过程中经历程序化基因组重排的一个反复出现的主题。