Post-transcriptional modifications and regulation of mRNAs in human mitochondria

Mitochondria contain their own circular genome (mtDNA), which encodes essential components of the oxidative phosphorylation (OXPHOS) system. Mitochondrial DNA transcription is a unique and relatively simple process, requiring a specialized transcription machinery that consists of a RNA polymerase (POLRMT), two transcription factors (TFAM and TFB2M), and an elongation factor (TEFM). During transcription, a non-canonical initiating nucleotide (NCIN) can be incorporated as the first nucleotide, serving as a 5' cap. Mitochondrial transcription produces large polycistronic transcripts, which are subsequently processed by ribonucleases to generate individual messenger RNAs (mt-mRNAs), ribosomal RNAs (mt-rRNAs), and transfer RNAs (mt-tRNAs). This review will specifically focus on the maturation and regulation of mt-mRNAs. Following their release from the primary transcript, mt-mRNAs undergo various post-transcriptional modifications, including methylation, pseudouridylation, and polyadenylation. These modifications play a crucial role in determining mt-mRNAs fate by influencing their stability, translation efficiency, and overall mitochondrial function. Additionally, the spatial organization of these processes within mitochondrial RNA granules (MRGs) suggests a compartmentalized system for mitochondrial gene regulation, ensuring precise coordination between transcription, processing, and translation. A deeper understanding of these post-transcriptional modifications provides valuable insights into mitochondrial gene expression and its broader impact on cellular metabolism.