On the Origin and Evolution of the Mitochondrial Ribosome

Abstract: The ribosome is among the most ancient, intricate and well studied macromolecular complexes in biology. Predating the earliest divergence of life, its core molecular structure has remained mostly unchanged for more than three billion years. In stark contrast to its monolithic ancestor, the mitochondrial ribosome represents one of the most architecturally diverse protein complexes investigated. This work is an attempt at reconciling these two paradigms. In this thesis I first briefly cover the evolutionary history of the mitochondrial ribosome: from its ancient origins, through the process of Eukaryogenesis and the development of mitochondria, to its current state. Following this I present a comprehensive and integrated comparative analysis of the current mitoribosomal structures. Using these structural observations as a starting point I then summarise the current knowledge regarding the evolutionary trends of mitochondrial ribosomes. Finally I review and discuss potential genetic mechanisms and evolutionary pressures which could have produced such a vibrant diversity of structures. Together with this analysis I present monosome structures from the ciliate Tetrahymena thermophila and chlorophycean Polytomella magna together with an assembly intermediate of the large subunit from Trypanosoma brucei. Together, I hope to demonstrate the impact of the unique mitochondrial environment on the evolution of the mitochondrial ribosome.