Mitochondria contain the genetic information and expression machinery to produce essential respiratory chain proteins. Within the mitochondrial matrix, newly synthesized RNA, RNA processing proteins and mitoribosome assembly factors form punctate sub-compartments referred to as mitochondrial RNA granules (MRGs)^1,2,3. Despite their proposed importance in regulating gene expression, the structural and dynamic properties of MRGs remain largely unknown. We investigated the internal architecture of MRGs using fluorescence super-resolution localization microscopy and correlative electron microscopy, and found that the MRG ultrastructure consists of compacted RNA embedded within a protein cloud. Using live-cell super-resolution structured illumination microscopy and fluorescence recovery after photobleaching, we reveal that MRGs rapidly exchange components and can undergo fusion, characteristic properties of fluid condensates⁴. Furthermore, MRGs associate with the inner mitochondrial membrane and their fusion coincides with mitochondrial remodelling. Inhibition of mitochondrial fission or fusion leads to an aberrant accumulation of MRGs into concentrated pockets, where they remain as distinct individual units despite their close apposition. Together, our findings reveal that MRGs are nanoscale fluid compartments, which are dispersed along mitochondria via membrane dynamics.

线粒体包含产生必需的呼吸链蛋白的遗传信息和表达机制。在线粒体基质中，新合成的RNA，RNA加工蛋白和线粒体装配因子形成点状小室，称为线粒体RNA颗粒（MRG）^1,2,3。尽管提出了它们在调节基因表达中的重要性，但是MRG的结构和动力学性质仍然很大程度上未知。我们使用荧光超分辨率定位显微镜和相关电子显微镜研究了MRG的内部结构，发现MRG的超微结构由嵌入蛋白质云中的紧密RNA组成。使用活细胞超分辨率结构照明显微镜和光漂白后的荧光恢复，我们发现MRGs可以快速交换组分，并且可以经历融合，流体冷凝物的特性⁴。此外，MRGs与内部线粒体膜相关联，其融合与线粒体重塑相吻合。线粒体裂变或融合的抑制导致MRGs异常积聚到集中的口袋中，尽管位置紧密，它们仍以不同的单个单位存在。总之，我们的发现表明MRGs是纳米级的流体隔室，通过膜动力学沿线粒体分散。