Structure-function relationships in mitochondrial transcriptional condensates

Phase separation organizes many membraneless structures in cells. The functional consequences of concentrating cellular machinery into biomolecular condensates, however, are largely unclear. One fundamental cellular function that has been linked to condensate formation is transcription. Here, we have reconstituted mitochondrial transcription in condensates from purified components. We find that the core components of the mttranscriptional machinery form multi-phasic, viscoelastic condensates in vitro. Strikingly, the rates of condensate-mediated transcription are substantially lower than equivalent reactions in bulk solution. These condensate-mediated decreases in transcriptional rates are associated with the formation of dynamically arrested vesicular structures that are driven by the production and accumulation of RNA during transcription. Using coarse-grained, equilibrium simulations, we show that the generation of RNA alters the phase behavior and the organization of transcriptional components within condensates and that the in vitro mtcondensates are non-equilibrium structures. Together, our in vitro and in silico approaches shed light on how proteins and (ribo)nucleic acids biophysically self-assemble within mitochondria in vivo. Our results highlight the complex morphologies of transcribing, multicomponent condensates and they illustrate the interdependent structure-function relationships in condensates. Significance StatementMitochondria condense their genome into transcriptionally active mt-nucleoids. These structures fit the definition of biomolecular condensates that form via macromolecular phase separation. We take advantage of the ability to reconstitute mitochondrial transcriptional condensates in vitro from minimal components. We find that the production and accumulation of RNA alters the phase behavior of transcriptional condensates. The altered phase behavior is linked to the formation of arrested, non-equilibrium vesicular structures. Similar changes to phase behavior of proteins and (ribo)nucleic acids can be recapitulated in live mitochondria through knockdown of mt-nucleoid core components. Computer simulations help identify biophysical mechanisms that are needed to maintain the steady-state structures of transcriptional condensates.