| dc.description | For this thesis, the organization, expression and evolutionary affiliations of the mitochondrial genome in the non-photosynthetic amoeboid protozoan, Acanthamoeba castellanii, have been investigated. A specific question that was posed was, "Is the mitochondrial genome of A. castellanii more closely affiliated with that of land plants or with that of the chlorophycean alga, Chlamydomonas reinhardtii?". (The latter share a recent common nuclear ancestor but differ strikingly in mitochondrial genome sequence and organization.) To answer this question, the DNA sequence of the A. castellanii mitochondrial rRNA genes and flanking regions has been determined. The LSU rRNA gene is flanked by two clusters of tRNA genes, cluster 1 (upstream) and cluster 2 (downstream), whereas the SSU rRNA gene is flanked by tRNA gene cluster 2 (upstream) and the cytochrome oxidase subunit I gene (downstream), with very little intergenic space and with all of these genes in the same transcriptional orientation. The tRNAs of cluster 2, and likely those of cluster 1, are altered by a novel type of RNA editing such that a conserved tRNA secondary structure is restored. The Acanthamoeba rRNA genes display primary sequence and secondary structure elements strikingly similar to those of their eubacterial homologs. In this regard, they resemble much more closely the mitochondrial rRNA genes of land plants than those of C. reinhardtii. Phylogenetic analysis suggests a monophyletic origin of mitochondria from within the $\alpha$-Proteobacteria. There are no intronic sequences within the SSU rRNA gene, but the LSU rRNA gene harbors three group I introns, all located within the 3$\sp\prime$ portion of the RNA. One of these introns is found at precisely the same nucleotide position as the group I intron of the LSU rRNA gene of C. reinhardtii chloroplast. These two introns appear to be structurally homologous, raising questions about their evolutionary origin. A cytochrome oxidase subunit 2 gene is located immediately downstream of the cytochrome oxidase subunit 1 gene and it is likely that these two genes are co-transcribed and, perhaps, co-translated. | en_US |
