Mitochondrial gene expression is a fundamental process for mitochondrial respiration. Several steps of mitochondrial gene expression appear to be spatially organized in two distinct structures containing both proteins and nucleic acids. The mitochondrial genome is present in hundreds of copies in mammalian cells, packed into structures named nucleoids. Once the mtDNA gets transcribed into long polycistronic RNAs, the newly synthesized mitochondrial RNAs form distinct foci, together with specific proteins, named mitochondrial RNA granules (MRGs). Different functions have been ascribed to MRGs, however to date there is still lack of information about the relevance of MRGs in mitochondrial gene expression and, more generally, in mitochondrial biology. Here, we present 3 complementary projects that aim at addressing the importance of MRGs by investigating: (1) novel MRG components; (2) the biophysical properties of MRGs, applying super-resolution microscopy; (3) proteins involved in the biogenesis and regulation of MRGs, using a high-throughput RNAi screen.