The endosymbiosis between the pathogenic fungus Rhizopus microsporus and the toxin-producing bacterium Mycetohabitans rhizoxinica represents a unique example of host control by an endosymbiont. Fungal sporulation strictly depends on the presence of endosymbionts as well as bacterially produced secondary metabolites. However, an influence of primary metabolites on host control remained unexplored. Recently, we discovered that M. rhizoxinica produces FO and 3PG-F420, a derivative of the specialized redox cofactor F420. Whether FO/3PG-F420 plays a role in the symbiosis has yet to be investigated. Here, we report that FO, the precursor of 3PG-F420, is essential to the establishment of a stable symbiosis. Bioinformatic analysis revealed that the genetic inventory to produce cofactor 3PG-F420 is conserved in the genomes of eight endofungal Mycetohabitans strains. By developing a CRISPR/Cas-assisted base editing strategy for M. rhizoxinica, we generated mutant strains deficient in 3PG-F420 (M. rhizoxinica ΔcofC) and in both FO and 3PG-F420 (M. rhizoxinica ΔfbiC). Co-culture experiments demonstrated that the sporulating phenotype of apo-symbiotic R. microsporus is maintained upon reinfection with wild-type M. rhizoxinica or M. rhizoxinica ΔcofC. In contrast, R. microsporus is unable to sporulate when co-cultivated with M. rhizoxinica ΔfbiC, even though the fungus was observed by super-resolution fluorescence microscopy to be successfully colonized. Genetic and chemical complementation of the FO deficiency of M. rhizoxinica ΔfbiC led to restoration of fungal sporulation, signifying that FO is indispensable for establishing a functional symbiosis. Even though FO is known for its light-harvesting properties, our data illustrate an important role of FO in inter-kingdom communication.

中文翻译：

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内共生细菌产生的去氮黄素代谢物控制真菌宿主繁殖

病原真菌小孢根霉和产毒素细菌Mycetohabitans rhizoxinica之间的内共生代表了内共生体控制宿主的独特例子。真菌孢子形成严格取决于内共生体以及细菌产生的次级代谢产物的存在。然而，初级代谢物对宿主控制的影响仍有待探索。最近，我们发现 M. rhizoxinica 产生 FO 和 3PG-F420，这是一种专门的氧化还原辅因子 F420 的衍生物。 FO/3PG-F420 是否在共生中发挥作用还有待研究。在这里，我们报道 FO，3PG-F420 的前体，对于建立稳定的共生至关重要。生物信息学分析表明，产生辅因子 3PG-F420 的遗传库存在八个内真菌 Mycetohabitans 菌株的基因组中是保守的。通过开发针对 M. rhizoxinica 的 CRISPR/Cas 辅助碱基编辑策略，我们生成了缺乏 3PG-F420 (M. rhizoxinica ΔcofC) 以及 FO 和 3PG-F420 (M. rhizoxinica ΔfbiC) 的突变菌株。共培养实验表明，无辅共生小孢子菌的孢子形成表型在用野生型根霉或根霉ΔcofC再感染后得以维持。相比之下，当与 M. rhizoxinica ΔfbiC 共培养时，R. microsporus 无法形成孢子，即使通过超分辨率荧光显微镜观察到该真菌成功定植。 M. rhizoxinica ΔfbiC 的 FO 缺陷的遗传和化学补充导致了真菌孢子形成的恢复，这表明 FO 对于建立功能性共生是必不可少的。尽管 FO 以其光捕获特性而闻名，但我们的数据说明了 FO 在王国间交流中的重要作用。