Overexpression of MTERFD1 or MTERFD3 impairs the completion of mitochondrial DNA replication

The physiological roles of the mitochondrial transcription termination factor (mTERF) family are poorly understood. MTERF and its homologues influence transcriptional readthrough in vitro, but the extent to which they regulate mitochondrial RNA levels in vivo is unclear. In addition, MTERF was previously shown to promote replication pausing. To test their roles in mtDNA metabolism, we created cell-lines inducibly expressing epitope-tagged versions of two members of the mTERF family, MTERFD1 and MTERFD3, as well as shRNA constructs targeted at each. We confirmed mitochondrial targeting and lack of sequence-specific DNA binding for both factors. Over-expression of epitope-tagged MTERFD1 or MTERFD3 resulted in modest mtDNA copy-number depletion and an accumulation of specific mtDNA replication intermediates indicating an impairment of the terminal steps of replication. These findings further implicate the mTERF family in restraining replication fork progression and support the idea that they facilitate the orderly passage of replication and transcription machineries, thus contributing to genome stability.