Ibaraki University College of Agriculture, Ami, Ibaraki, Japan.
Center for Conservation Science, Tokyo National Research Institute for Cultural Properties, Tokyo, Japan.
Appl Environ Microbiol. 2020 Sep 1;86(18). doi: 10.1128/AEM.01018-20.
Obligate bacterial endosymbionts are critical to the existence of many eukaryotes. Such endobacteria are usually characterized by reduced genomes and metabolic dependence on the host, which may cause difficulty in isolating them in pure cultures. Family -related endofungal bacteria affiliated with the clade can be associated with the fungal subphyla Mortierellomycotina and Glomeromycotina. In this study, a cultivable endosymbiotic bacterium, sp. strain B2-EB, present in the fungal host was obtained successfully. The B2-EB genome (1.88 Mb) represents the smallest genome among the endofungal bacterium (2.64-2.80 Mb) of and the uncultured endosymbiont " Glomeribacter gigasporarum" (1.37 to 2.36 Mb) of arbuscular mycorrhizal fungi. Despite a reduction in genome size, strain B2-EB displays a high genome completeness, suggesting a nondegenerative reduction in the B2-EB genome. Compared with a large proportion of transposable elements (TEs) in other known genomes (7.2 to 11.5% of the total genome length), TEs accounted for only 2.4% of the B2-EB genome. This pattern, together with a high proportion of single-copy genes in the B2-EB genome, suggests that the B2-EB genome reached a state of relative evolutionary stability. These results represent the most streamlined structure among the cultivable endofungal bacteria and suggest the minimal genome features required by both an endofungal lifestyle and artificial culture. This study allows us to understand the genome evolution of -related endosymbionts and to elucidate microbiological interactions. This study attempted the isolation of a novel endobacterium, sp. B2-EB (JCM 33615), harbored in the fungal host E1425 (JCM 39028). We report the complete genome sequence of this strain, which possesses a reduced genome size with relatively high genome completeness and a streamlined genome structure. The information indicates the minimal genomic features required by both the endofungal lifestyle and artificial cultivation, which furthers our understanding of genome reduction in fungal endosymbionts and extends the culture resources for biotechnological development on engineering synthetic microbiomes.
专性细菌内共生体对于许多真核生物的存在至关重要。这种内细菌通常具有缩小的基因组和对宿主的代谢依赖性，这可能导致难以在纯培养物中分离它们。与真菌子囊菌亚门和球囊菌亚门相关的真菌内相关细菌可与真菌子囊菌亚门和球囊菌亚门相关。在这项研究中，成功获得了存在于真菌宿主中的可培养共生细菌，sp. 株 B2-EB。B2-EB 基因组（1.88 Mb）代表了真菌内细菌中最小的基因组（2.64-2.80 Mb）和未培养的共生体“Glomeribacter gigasporarum”（1.37 至 2.36 Mb）的最小基因组。尽管基因组大小减小，但菌株 B2-EB 显示出高基因组完整性，表明 B2-EB 基因组的非退化性减小。与其他已知的基因组（7.2%至 11.5%的总基因组长度）中大量转座元件（TEs）相比，TEs 仅占 B2-EB 基因组的 2.4%。这种模式，再加上 B2-EB 基因组中单拷贝基因的高比例，表明 B2-EB 基因组达到了相对进化稳定的状态。这些结果代表了可培养真菌内细菌中最简化的结构，并表明了内共生生活方式和人工培养所需的最小基因组特征。本研究使我们能够了解相关内共生体的基因组进化，并阐明微生物相互作用。本研究试图分离一种新型内细菌，sp. B2-EB（JCM 33615），栖息在真菌宿主 E1425（JCM 39028）中。我们报告了该菌株的完整基因组序列，该菌株具有缩小的基因组大小，相对较高的基因组完整性和简化的基因组结构。这些信息表明了内共生生活方式和人工培养所需的最小基因组特征，这进一步加深了我们对真菌内共生体基因组缩小的理解，并扩展了用于工程合成微生物组的生物技术发展的培养资源。
