A novel model of phase separation deriving mitochondrial genome assembly and transcription

Mitochondria are the powerhouse of mammalian cells, which also play critical roles in cell metabolism, cell signal and apoptosis. They are the only organelles containing own DNA, which is compacted into a special sub-organelle as nucleoid. What’s more, the nucleoid is not only a structure for genetic material storage, but also responsible for mitochondrial DNA (mtDNA) replication and transcription. However, the mechanism of nucleoid assembling and transcription regulation is poorly understood.  

In a study published in Nature Structural & Molecular Biology, LIU Xingguo's group from Guangzhou Institutes of Biomedicine and Health of the Chinese Academy of Sciences proposed a model of mitochondrial nucleoid self-assembly driven by phase separation which is a new pattern of recruitment of the mitochondrial transcription machinery via co-phase separation.  

The researchers identified that mitochondrial transcription factor A (TFAM) undergoes phase separation driven by intrinsically disordered regions (IDRs), and phase separation of TFAM and DNA drives nucleoid formation.  

To illustrate the relationship between nucleoid assembling and mitochondrial transcription, they assembled the transcription machineries in vitro. They found that nucleoid droplet formation promotes recruitment of the transcription machinery via a special, co-phase separation that concentrates transcription initiation, elongation and termination factors, and retains substrates to facilitate mtDNA transcription. They also found a special multi-phase separation of POLRMT, the RNA polymerase for mitochondrial DNA. POLRMT is a ring-like structure surrounding mtDNA in vitro and in cells.  

Furthermore, the researchers found that the elongation factor TEFM interrupts the multi-phase separation and brings more POLRMT in the inner phase of nucleoid containing mtDNA-TFAM and promotes the transcription, and they also observed the multi-phase separation of the termination factor MTERF1 with nucleoid in transcription termination.  

This study proposed a model of mitochondrial nucleoid self-assembly driven by phase separation of TFAM, and a new pattern of co-phase separation involved in mitochondrial transcriptional regulation, which orchestrates the roles of TFAM in both mitochondrial nucleoid organization.