<i>Chromulinavorax destructans</i>, a pathogen of microzooplankton that provides a window into the enigmatic candidate phylum Dependentiae

Members of the major candidate phylum Dependentiae (a.k.a. TM6) are widespread across diverse environments from showerheads to peat bogs; yet, with the exception of two isolates infecting amoebae, they are only known from metagenomic data. The limited knowledge of their biology indicates that they have a long evolutionary history of parasitism. Here, we present Chromulinavorax destructans (Strain SeV1) the first isolate of this phylum to infect a representative from a widespread and ecologically significant group of heterotrophic flagellates, the microzooplankter Spumella elongata (Strain CCAP 955/1). Chromulinavorax destructans has a reduced 1.2 Mb genome that is so specialized for infection that it shows no evidence of complete metabolic pathways, but encodes an extensive transporter system for importing nutrients and energy in the form of ATP from the host. Its replication causes extensive reorganization and expansion of the mitochondrion, effectively surrounding the pathogen, consistent with its dependency on the host for energy. Nearly half (44%) of the inferred proteins contain signal sequences for secretion, including many without recognizable similarity to proteins of known function, as well as 98 copies of proteins with an ankyrin-repeat domain; ankyrin-repeats are known effectors of host modulation, suggesting the presence of an extensive host-manipulation apparatus. These observations help to cement members of this phylum as widespread and diverse parasites infecting a broad range of eukaryotic microbes.从属菌门（Dependentiae，又名TM6）作为一类主要的候选菌门，其成员广泛分布于从淋浴喷头到泥炭沼等多种生境中；然而，除两株感染变形虫的分离株外，人们仅通过宏基因组数据（metagenomic data）知晓该类群。目前对其生物学特性的有限认知表明，该类群拥有漫长的寄生演化历史。本研究报道了破坏色质杆状菌（Chromulinavorax destructans，菌株SeV1），它是该门首个可感染一类分布广泛且生态意义重要的异养鞭毛虫（heterotrophic flagellates）类群代表株的分离株，该宿主为微型浮游动物（microzooplankter）长形泡沫虫（Spumella elongata，菌株CCAP 955/1）。破坏色质杆状菌拥有缩减至1.2 Mb的基因组，其高度特化以适应寄生感染：完整代谢通路的痕迹完全缺失，但编码了一套庞大的转运系统，用于从宿主处获取营养物质以及以ATP（三磷酸腺苷）形式存在的能量。该菌的增殖会引发宿主线粒体发生大规模重组与扩增，最终将病原菌完全包裹，这与其依赖宿主获取能量的特性相符。在其预测编码的蛋白质中，近半数（44%）带有分泌信号序列，其中包括大量与已知功能蛋白无明显同源性的蛋白，以及98个带有锚蛋白重复结构域（ankyrin repeat domain）的蛋白拷贝；锚蛋白重复序列是已知的宿主调控效应因子，这暗示该菌拥有一套复杂的宿主操控系统。上述研究结果进一步证实，从属菌门类群是一类分布广泛、种类多样的寄生虫，可感染多种真核微生物。