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首页> 外文期刊>Proceedings of the National Academy of Sciences of the United States of America >Complete nucleotide sequence of the chlorarachniophyte nucleomorph: Nature's smallest nucleus
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Complete nucleotide sequence of the chlorarachniophyte nucleomorph: Nature's smallest nucleus

机译：绿藻植物核形态的完整核苷酸序列：自然界最小的核

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The introduction of plastids into different heterotrophic protists created lineages of algae that diversified explosively, proliferated in marine and freshwater environments, and radically altered the biosphere. The origins of these secondary plastids are usually inferred from the presence of additional plastid membranes. However, two examples provide unique snapshots of secondary-endosymbiosis-in-action, because they retain a vestige of the endosymbiont nucleus known as the nucleomorph. These are chlorarachniophytes and cryptomonads, which acquired their plastids from a green and red alga respectively. To allow comparisons between them, we have sequenced the nucleomorph genome from the chlorarachniophyte Bigelowiella natans: at a mere 373,000 by and with only 331 genes, the smallest nuclear genome known and a model for extreme reduction. The genome is eukaryotic in nature, with three linear chromosomes containing densely packed genes with numerous overlaps. The genome is replete with 852 introns, but these are the smallest introns known, being only 18, 19, 20, or 21 nt in length. These pygmy introns are shown to be miniaturized versions of normal-sized introns present in the endosymbiont at the time of capture. Seventeen nucleomorph genes encode proteins that function in the plastid. The other nucleomorph genes are housekeeping entities, presumably underpinning maintenance and expression of these plastid proteins. Chlorarachniophyte plastids are thus serviced by three different genomes (plastid, nucleomorph, and host nucleus) requiring remarkable coordination and targeting. Although originating by two independent endosymbioses, chlorarachniophyte and cryptomonad nucleomorph genomes have converged upon remarkably similar architectures but differ in many molecular details that reflect two distinct trajectories to hypercompaction and reduction.
机译：将质体引入不同的异养生物中，产生了藻类谱系，这些谱系爆炸性地多样化，在海洋和淡水环境中增殖，并从根本上改变了生物圈。这些次要质体的起源通常是从附加质体膜的存在来推断的。但是，有两个例子提供了作用中次要内生共生的独特快照，因为它们保留了共生核内核的痕迹，称为核型。它们分别是绿藻和隐藻，它们分别从绿藻和红藻获得质体。为了让它们之间进行比较，我们对来自拟南芥（Bigarwiwiella natans）的拟南芥的核型基因组进行了测序：仅由373,000个基因组成，仅有331个基因，这是已知的最小的核基因组，并且是一个极度减少的模型。该基因组在本质上是真核生物，具有三个线性染色体，其中包含紧密堆积的基因，具有许多重叠。基因组中充满了852个内含子，但它们是已知的最小内含子，长度仅为18、19、20或21 nt。这些侏儒内含子显示为捕获时内共生体中存在的正常大小内含子的微型化形式。十七个核型基因编码在质体中起作用的蛋白质。其他核型基因是管家实体，大概是这些质体蛋白维持和表达的基础。因此，绿藻类植物质体由三个不同的基因组（质体，核型和宿主核）提供服务，需要显着的协调和靶向作用。尽管由两个独立的内共生酶起源，但绿藻纲植物和隐孢子虫的核型基因组已经汇聚在非常相似的结构上，但是在许多分子细节上有所不同，反映了超紧缩和还原的两个不同轨迹。

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