| dc.description | Mitochondria, organelles found in eukaryotic cells, are responsible for ATP production as well as a suite of other cellular functions. All eukaryotic cells are now believed to contain either a mitochondrion or a related organelle. Most mitochondria contain a remnant genome, an apparent relict of the mitochondrion's ancestry as a free-living alpha-proteobacterium. Though highly reduced, the mitochondrial genome is maintained and, as such, the regulation of its expression and replication is critical. Notably, rather than using bacteria-derived genes, as would be expected a priori, the genes involved in mitochondrial transcription and replication are, in fact, related to bacteriophage genes. In this thesis, the evolution of a number of proteins involved in mitochondrial transcription and replication has been examined by obtaining and comparing the corresponding gene sequences from representative eukaryotes. These proteins include the single-subunit RNA polymerase (ssPNAP), mitochondrial transcription factor B (mtTFB) and the mitochondrial helicase (Twinkle). Most mitochondria, with the possible exception of those found in the jakobids (a little-studied group of flagellated protozoa), use a nucleus-encoded ssRNAP similar to the one encoded by the T7 bacteriophage. To answer questions regarding the origin of the ssRNAP, ten novel ssRNAP sequences were obtained and analyzed. A mitochondrial transcription factor, mtTFB, derived from an rRNA dimethyladenosine transferase (DMT), had previously been identified only in the opisthokonts (animals, fungi and their specific unicellular relatives). Extensive searches suggest that homologs of mtTFB are limited to Opisthokonta, Amoebozoa (amoeboid protists) and trypanosomatids. Furthermore, phylogenetic analyses show a specific affinity of mtTFB homologs to the DMTs of alpha-proteobacteria, supporting an endosymbiont origin. In addition, the phylogenetic distribution of another phage-derived gene, Twinkle, was investigated. Twinkle appears to be the mitochondrial primase in most eukaryotes, with the possible exception of Metazoa. Finally, data linking the mitochondrial ssRNAP and Twinkle proteins to components encoded by T7-related phage are discussed with regard to the origin of the mitochondrial DNA replication machinery. I posit that the ssMAP was obtained initially from a free-living bacteriophage for use in priming DNA replication, rather than for transcription per se. | en_US |
