Functional polypeptides can be synthesized from human mitochondrial transcripts lacking termination codons

The human mitochondrial genome (mtDNA) is a small, circular DNA duplex found in multi-copy in the mitochondrial matrix. It is almost fully transcribed from both strands to produce large polycistronic RNA units that are processed and matured. The 13 mtDNA-encoded polypeptides are translated from mt-mRNAs that have been matured by polyadenylation of their free 3´-termini. A patient with clinical features consistent with an mtDNA disorder was recently shown to carry a microdeletion, resulting in the loss of the termination codon for MTATP6 and in its juxtaposition with MTCO3. Cell lines from this patient exhibited low steady-state levels of RNA14, the bi-cistronic transcript encoding subunits 6 and 8 of the FoF1-ATP synthase, complex V, consistent with a decreased stability. Recent reports of ‘non-stop’ mRNA decay systems in the cytosol have failed to determine the fate of gene products derived from transcripts lacking termination codons, although enhanced decay clearly required the ‘non-stop’ transcripts to be translated. We wished to determine whether functional translation products could still be expressed from non-stop transcripts in the human mitochondrion. Although a minor defect in complex V assembly was noted in the patient-derived cell lines, the steady-state level of ATPase 6 was similar to controls, consistent with the pattern of de novo mitochondrial protein synthesis. Moreover, no significant difference in ATP synthase activity could be detected. We conclude that, in the absence of a functional termination codon, although mitochondrial transcripts are more rapidly degraded, they are also translated to generate stable polypeptides that are successfully integrated into functional enzyme complexes.