By analyzing the genome-wide microRNA (miRNA) expression profile using microRNA microarray and stem-loop miRNA qPCR assay, we report that unique miRNAs are enriched in mitochondria and these mitochondria-associated miRNAs may be involved in the regulation of gene expression of mitochondria and other general cellular processes such as apoptosis, proliferation and differentiation.

MiRNAs have been shown to play an essential role in modulating gene expression through translational inhibition and/or degradation of target mRNAs. As a new class of endogenous non-coding RNAs, miRNAs generally act to negatively regulate gene expression at the post-transcriptional level by base-pairing to complementary sites on the target mRNAs, thereby blocking translation or causing the degradation of the target mRNA 1. The regulation of miRNAs generally takes place in the cytosol following miRNA-loaded RNA-induced silencing complex binding to the target mRNA 2. Although certain mature miRNAs have been found in the nucleus 3 and the specialized processing (P) bodies of the cell cytoplasm 4, it remains unknown whether subcellular organelles such as mitochondria are associated with specific miRNAs. Since mitochondria are important subcellular organelles that contain proteins translated from mRNA transcripts derived from either the nuclear or their own genome 5, they may provide a potential site for miRNA-mediated post-transcriptional regulation. Very recently, Kren et al. 6 reported the detection of 15 nuclear-coded miRNAs in rat liver mitochondria. This initial study, however, did not reveal the specific enrichment of miRNAs in mitochondria. So far, there is no report about mitochondrial miRNAs in animal tissues other than liver.

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