A new series of in vivo experiments has discredited the long-held hypothesis that protein mitochondrial transcription termination factor 1 (MTERF1) regulates the synthesis of mitochondrial ribosomal DNA (rDNA). Although this hypothesis was based on decades of in vitro studies, the new data show that, while MTERF1 does keep mitochondrial DNA (mtDNA) transcription proceeding smoothly, it’s not required for the direct regulation of rRNA synthesis.
Previous studies on MTERF1 have been limited by the fact that fully knocking out the protein poses technical challenges. Two copies of the MTERF1 gene exist on the same chromosome, close enough that recombination of both copies is difficult. So experiments typically have centered on biochemical approaches to probe the function of isolated MTERF1 protein.
Those studies suggested that MTERF1 binds to mitochondrial rRNA genes. In humans mtDNA exists as a circular chromosome containing 37 genes. The two strands of the mtDNA are labeled, based on nucleotide content, as the heavy strand and the light strand—each strand encodes a separate set of genes. Based on the in vitro data, it was thought that MTERF1 binds to the heavy strand, which contains the rRNA genes, playing a key regulatory role by stopping transcription at the correct spots.
But when Terzioglu and colleagues finally created an MTERF1 knockout mouse, they observed no effect on mitochondrial rRNA transcription levels. “Our surprising findings started at this point,” said Terzioglu, “and we decided to go on and study the detailed mechanism and in vivo biological function of the MTERF1 protein.”
Through a series of experiments that spanned nearly ten years, the researchers found that MTERF1 doesn’t bind to the mtDNA heavy strand but rather to the light strand at a site opposite the gene of interest. It blocks transcription on that segment of the light chain to allow clearance for the transcription machinery to proceed smoothly along the heavy strand (1).
The new role of MTERF1 as a traffic director “changes all our basic knowledge on mitochondrial metabolism,” Terzioglu said. As mitochondria are the energy-generating powerhouses of cells and contain genes key to this function, understanding the regulation of mtDNA transcription can shed light on how cellular metabolism as a whole is regulated.
“If we fully understand the biological functions of all MTERF family members, this will bring us more insights to better understand some mitochondrial diseases,” said Terzioglu.
The findings illustrate the nature of science, she added, with hypotheses constantly being reworked as new data becomes available. “Biology has its own laws, and sometimes it is simpler than what we are thinking,” she said.
- Terzioglu, M., B. Ruzzenente, J. Harmel, A. Mourier, E. Jemt, et. al. 2013. MTERF1 Binds mtDNA to prevent transcriptional interference at the light-strand promoter but is dispensable for rRNA gene transcription regulation. Cell Metabolism 17. http://dx.doi.org/10.1016/j.cmet.2013.03.006.