Transcription-dependent phase coexistence of mitochondrial nucleoids and RNA granules.

The spatiotemporal organization of multiple components within biomolecular condensates helps coordinate gene expression. Mitochondria separate transcription and RNA processing into two distinct condensates: mt-nucleoids and mtRNA granules (MRGs), respectively. However, how mtRNA transcripts are transferred from mt-nucleoids to distant MRGs was unclear. With high-resolution imaging, we examined the steady-state organization of mt-condensates in human cells. We identified a wide distribution of distances between centroids of mt-condensates, from roughly a micron apart to within 100 nm of each other. Live imaging revealed that such organization was dynamic: mt-condensates frequently underwent cycles of mixing and demixing. Indeed, mtRNA transcripts and the mtRNA polymerase co-localized within mixed mt-condensates, while transcription inhibition led to complete dissolution of MRGs, supporting that nascently transcribed mtRNA is a key driver of mt-condensate organization. Together, our results show that active transcription sustains the phase coexistence between mt-nucleoids and MRGs, with implications for transcriptional condensates more broadly.