Final Activity Report Summary - CTTFMM (Characterization of transcription termination factors in mammalian mitochondria)
Human mitochondria contain multiple copies of a small double stranded deoxyribonucleic acid (DNA) genome (mtDNA), of approximately 16.6 kb size, which encodes for two ribosomal ribonucleic acids (RNAs), a set of 22 transfer RNAs and 13 proteins involved in respiration. Gene expression in mitochondria is not self-sufficient since nuclear genes encode all protein components that are required for transcription and translation of the mtDNA-encoded genes as well as all proteins necessary for mtDNA replication. In human cells each of the strands in mtDNA contains one major promoter for transcriptional initiation, the light-strand promoter (LSP) and the heavy-strand promoter (HSP). Transcription from LSP and HSP produces polycistronic precursor RNA encompassing all the genetic information encoded in each of the specific strands.
In this Marie Curie financed project, we initiated work to investigate how mitochondrial gene expression was regulated in response to pathological processes and metabolic demands. Proper regulation of mtDNA expression was also likely to require factors that could directly regulate mitochondrial transcription initiation in response to decreased needs for oxidative phosphorylation capacity. We identified a novel family of genes that appeared to play this role in mammalian cells.
The mitochondrial transcription termination factor (MTERF) proteins, namely MTERF2, MTERF3, and MTERF4 had the capacity to regulate mitochondrial transcription in vivo. We characterised these factors in a series of unpublished investigations and found that they played a key role in the integration of nuclear and mitochondrial gene expression.
The first results of this project were published by Park et al. 2007, 130(2):273-85. In this study, we showed that MTERF3 was a negative regulator of mtDNA transcription initiation. The MTERF3 gene was essential, as homozygous knockout mouse embryos died in mid-gestation. Tissue-specific inactivation of MTERF3 in the heart caused aberrant mtDNA transcription and severe respiratory chain deficiency. MTERF3 bound the mtDNA promoter region and depletion of MTERF3 increased transcription initiation on both mtDNA strands. This increased transcription initiation led to decreased expression of critical promoter-distal tRNA genes, possibly explained by transcriptional collision on the circular mtDNA molecule. MTERF3 was the first example of a mitochondrial protein that acted as a specific repressor of mammalian mtDNA transcription initiation in vivo.
In future work, we intended to continue to investigate the molecular and physiological role of the MTERF family of proteins. We also hoped to identify specific substances which could regulate MTERF activity and thereby influence the metabolic activity of mammalian cells.
In this Marie Curie financed project, we initiated work to investigate how mitochondrial gene expression was regulated in response to pathological processes and metabolic demands. Proper regulation of mtDNA expression was also likely to require factors that could directly regulate mitochondrial transcription initiation in response to decreased needs for oxidative phosphorylation capacity. We identified a novel family of genes that appeared to play this role in mammalian cells.
The mitochondrial transcription termination factor (MTERF) proteins, namely MTERF2, MTERF3, and MTERF4 had the capacity to regulate mitochondrial transcription in vivo. We characterised these factors in a series of unpublished investigations and found that they played a key role in the integration of nuclear and mitochondrial gene expression.
The first results of this project were published by Park et al. 2007, 130(2):273-85. In this study, we showed that MTERF3 was a negative regulator of mtDNA transcription initiation. The MTERF3 gene was essential, as homozygous knockout mouse embryos died in mid-gestation. Tissue-specific inactivation of MTERF3 in the heart caused aberrant mtDNA transcription and severe respiratory chain deficiency. MTERF3 bound the mtDNA promoter region and depletion of MTERF3 increased transcription initiation on both mtDNA strands. This increased transcription initiation led to decreased expression of critical promoter-distal tRNA genes, possibly explained by transcriptional collision on the circular mtDNA molecule. MTERF3 was the first example of a mitochondrial protein that acted as a specific repressor of mammalian mtDNA transcription initiation in vivo.
In future work, we intended to continue to investigate the molecular and physiological role of the MTERF family of proteins. We also hoped to identify specific substances which could regulate MTERF activity and thereby influence the metabolic activity of mammalian cells.