The might of microRNA in mitochondria.

The mitochondrion is endowed with all the elements necessary for protein synthesis: it harbors its own genome—in the guise of a circular 16.5-kb chromosome— and transcription and translation apparatus. However, mitochondrial DNA (mtDNA) has just 37 genes, which encode 22 mitochondrial tRNAs, 2 mitochondrial rRNAs, and only 13 protein subunits belonging to respiratory complexes I, III, IV, and V1; all the subunits of complex II and the 1000 other proteins needed within the mitochondrion for its proper functioning are transcribed from nuclear genes, synthesized in the cytosol, and then transported into the organelle, where many are then posttranslationally processed before being located into position. The limited size of mtDNA also means that most of the machinery needed to regulate mitochondrial gene expression derives from nuclear genes.2 To date, the mechanisms underlying the coordinated expression of mitochondrial-encoded and nuclear gene–encoded transcripts remain to be fully elucidated.