Mitochondrial mRNA fragments are circularized in a human HEK cell line.

The relatively recent focus on the widespread occurrence and abundance of circular RNAs (circRNA) in the human cell nucleus has sparked an intensive interest in their existence and possible roles in cell gene expression and physiology. The presence of circRNAs in mammalian mitochondria, however, has been under-explored. Mitochondrial mRNAs differ from those produced from nuclear genes because they lack introns and are transcribed as poly-cistronic transcripts that are endonucleolytically cleaved, leaving transcripts with very small 5' and 3' UTRs.