Molecular mechanisms for transcription in mammalian mitochondria

The circular double stranded mitochondrial genome (mtDNA), which is about 16,600 bp in humans and 16,300 bp in mice, encodes 13 of the -90 different proteins present in the respiratory chain of mammalian mitochondria. The remaining components of the respiratory chain are encoded by nuclear genes and imported into the mitochondrial network. The genes present in mtDNA, 13 mRNAs, 22 tRNAs, and 2 rRNAs, are all essential for oxidative phosphorylation. Therefore, mtDNA replication and transcription are necessary processes for normal function of the respiratory chain and for the metabolism of the eukaryotic cell. In this thesis, we characterized the basic mtDNA transcription machinery in mammals. We identified two novel mitochondrial transcription factors, B1 (TFB1M) and B2 (TFB2M) and for the first time we reconstituted mammalian mitochondrial transcription in vitro by using only pure recombinant proteins. We showed that either TFB1M or TFB2M can support promoter-specific mtDNA transcription in vitro if combined with mitochondrial RNA polymerase (POLRMT) and mitochondrial transcription factor A (TFAM). Studies by us and others suggest that TFB1M and TFB2M may have distinct functions: TFB2M may be primarily a transcription factor and TFB1M a methyltransferase. We identified homologues to TFB1M and TFB2M in many metazoans, including Drosophila melanogaster, indicating a duplication event of the TFBM gene early in evolution. We used the recombinant in vitro transcription system to investigate molecular mechanisms for mtDNA promoter recognition in mammals. The transcription machineries reconstituted from mouse and human cells do not recognize the light strand promoter from the other species. By swapping transcription factors between the mouse and the human transcription machineries, we demonstrated that the observed promoter specificity is governed by POLRMT and TFAM. In contrast, TFB2M does not influence the sequence specificity in these two mammalian systems. TFAM is a dual function protein. The protein binds without ...