miRNAs in mtDNA-less cell mitochondria

The novel regulation mechanism in mtDNA-less cells was investigated. Very low mtDNA copy in mtDNA-less 206 rho degrees cells was identified. But no 13 mitochondria-specific proteins were translated in 206 rho degrees cells. Their mitochondrial respiration complexes V, III and II were 86.5, 29.4 and 49.6% of 143B cells, respectively. Complexes I and IV completely lack in 206 rho degrees cells. Non-mitochondrial respiration to generate ATP in 206 rho degrees cells was discovered. The expression levels of some mitochondrial RNAs including 12S rRNA, COX1, COX2, COX3, ND4 and ND5 were low. However, ND1, ND3 and Cyto b were not expressed in 206 rho degrees cells. Unequal transcription of mitochondria! RNAs indicated the post-transcriptional cleavage and processing mechanisms in the regulation of mitochondrial gene expression in 206 rho degrees cells. MicroRNAs (miRNAs) may modulate these mitochondrial RNA expression in these cells. RNA-induced silencing complex indeed within 206 rho degrees cell mitochondria indicated miRNAs in 206 rho degrees cell mitochondria. miRNA profile in mtDNA-less 206 rho degrees cells was studied by next-generation sequencing of small RNAs. Several mitochondria-enriched miRNAs such as miR-181c-5p and miR-146a-5p were identified in 206 rho degrees cell mitochondria. miR-181c-5p and miR-146a-5p had 23 and 19 potential targets on mitochondrial RNAs respectively, and these two miRNAs had multiple targets on mitochondria-associated messenger RNAs encoded by nuclear genes. These data provided the first direct evidence that miRNAs were imported into mitochondria and regulated mitochondrial RNA expressions.