Most secondary nonphotosynthetic eukaryotes have retained residual plastids whose physiological role is often still unknown. One such example is Euglena longa, a close nonphotosynthetic relative of Euglena gracilis harboring a plastid organelle of enigmatic function. By mining transcriptome data from E. longa, we finally provide an overview of metabolic processes localized to its elusive plastid. The organelle plays no role in the biosynthesis of isoprenoid precursors and fatty acids and has a very limited repertoire of pathways concerning nitrogen-containing metabolites. In contrast, the synthesis of phospholipids and glycolipids has been preserved, curiously with the last step of sulfoquinovosyldiacylglycerol synthesis being catalyzed by the SqdX form of an enzyme so far known only from bacteria. Notably, we show that the E. longa plastid synthesizes tocopherols and a phylloquinone derivative, the first such report for nonphotosynthetic plastids studied so far. The most striking attribute of the organelle could be the presence of a linearized Calvin-Benson (CB) pathway, including RuBisCO yet lacking the gluconeogenetic part of the standard cycle, together with ferredoxin-NADP⁺ reductase (FNR) and the ferredoxin/thioredoxin system. We hypothesize that the ferredoxin/thioredoxin system activates the linear CB pathway in response to the redox status of the E. longa cell and speculate on the role of the pathway in keeping the redox balance of the cell. Altogether, the E. longa plastid defines a new class of relic plastids that is drastically different from the best-studied organelle of this category, the apicoplast.

中文翻译：

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Euglena longa 的隐性质体定义了一种新型的非光合质体细胞器

大多数次级非光合真核生物保留了残留的质体，其生理作用通常仍然未知。一个这样的例子是Euglena longa，Euglena gracilis 的一种近亲非光合作用的近亲，含有一种具有神秘功能的质体细胞器。通过从E. longa挖掘转录组数据，我们最终提供了定位于其难以捉摸的质体的代谢过程的概述。细胞器在类异戊二烯前体和脂肪酸的生物合成中没有作用，并且关于含氮代谢物的途径非常有限。相比之下，磷脂和糖脂的合成被保留了下来，奇怪的是，磺基奎诺糖基二酰基甘油合成的最后一步是由 SqdX 形式的一种酶催化的，迄今为止仅从细菌中得知。值得注意的是，我们表明E. longaplastid 合成生育酚和叶绿醌衍生物，这是迄今为止研究的第一个非光合质体的此类报告。细胞器最显着的属性可能是存在线性化的 Calvin-Benson (CB) 通路，包括 RuBisCO 但缺乏标准循环的糖异生部分，以及铁氧还蛋白-NADP ⁺还原酶 (FNR) 和铁氧还蛋白/硫氧还蛋白系统. 我们假设铁氧还蛋白/硫氧还蛋白系统激活线性 CB 通路以响应E. longa细胞的氧化还原状态，并推测该通路在保持细胞氧化还原平衡方面的作用。总之，E. longa plastid 定义了一类新的遗物质体，它与这一类别中研究最多的细胞器 apicoplast 截然不同。