Regulator of post-transcriptional mitochondrial gene expression, required for assembly of the mitochondrial ribosome and for recruitment of mRNA and lncRNA. Binds RNAs containing the 14 base G-rich element. Preferentially binds RNAs transcribed from three contiguous genes on the light strand of mtDNA, the ND6 mRNA, and the long non-coding RNAs for MT-CYB and MT-ND5, each of which contains multiple consensus binding sequences (PubMed:23473033, PubMed:23473034, PubMed:29967381). Involved in the degradosome-mediated decay of non-coding mitochondrial transcripts (MT-ncRNA) and tRNA-like molecules (PubMed:29967381). Acts by unwinding G-quadruplex RNA structures in MT-ncRNA, thus facilitating their degradation by the degradosome (PubMed:29967381). G-quadruplexes (G4) are non-canonical 4 stranded structures formed by transcripts from the light strand of mtDNA (PubMed:29967381). Monomer. Found in a complex with DDX28, DHX30, FASTKD2 and FASTKD5 (PubMed:25683715). Interacts with the mitochondrial RNase P complex subunit TRMT10C/MRPP1 (PubMed:23473034). Interacts with the 2 components of the mitochondrial degradosome complex, PNPT1 and SUPV3L1, in an RNA-dependent manner (PubMed:29967381). Localizes to mitochondrial RNA granules found in close proximity to the mitochondrial nucleoids. The RRM domains mediate RNA-binding. Depletion of GRSF1 by siRNA results in a combined OXPHOS assembly defect, with the prominent loss of complexes I, III, IV, and V. It also leads to altered steady-state levels of mitochondrial rRNAs and mRNAs. Unlikely isoform. Aberrant splice sites. Intron retention. Unlikely isoform. Aberrant splice sites. Unlikely isoform. Aberrant splice sites.