Abstract
Mitochondria are the powerhouses of eukaryotic cells. They are considered as semi-autonomous because they have retained genomes inherited from their prokaryotic ancestor and host fully functional gene expression machineries. These organelles have attracted considerable attention because they combine bacterial-like traits with novel features that evolved in the host cell. Among them, mitochondria use many specific pathways to obtain complete and functional sets of tRNAs as required for translation. In some instances, tRNA genes have been partially or entirely transferred to the nucleus and mitochondria require precise import systems to attain their pool of tRNAs. Still, tRNA genes have also often been maintained in mitochondria. Their genetic arrangement is more diverse than previously envisaged. The expression and maturation of mitochondrial tRNAs often use specific enzymes that evolved during eukaryote history. For instance many mitochondria use a eukaryote-specific RNase P enzyme devoid of RNA. The structure itself of mitochondrial encoded tRNAs is also very diverse, as e.g., in Metazoan, where tRNAs often show non canonical or truncated structures. As a result, the translational machinery in mitochondria evolved adapted strategies to accommodate the peculiarities of these tRNAs, in particular simplified identity rules for their aminoacylation. Here, we review the specific features of tRNA biology in mitochondria from model species representing the major eukaryotic groups, with an emphasis on recent research on tRNA import, maturation and aminoacylation.
Highlights
The understanding of mitochondrial biology has important societal implications, primarily because mitochondrial dysfunctions often result in serious disorders such as myopathies or other neuro-degenerative diseases in human [1]
The observed steady-state levels of imported tRNAs correlate with the occurrence frequencies of the cognate codons for both mitochondrial and nuclear genes. This fine-tuning between tRNA import and the codon usage in Chlamydomonas seems to be the result of a co-evolutive process rather than a dynamic adaptation of cytosolic tRNA import into mitochondria [68]
At the mt-surface, the tRNA is loaded on pre-MSK1 that is synthesized at the mt-surface and enables the co-import of the tRNA via the protein import machinery while Eno2p is incorporated in the glycolytic complex associated to the mt-surface [57,69,70]
Summary
The understanding of mitochondrial biology has important societal implications, primarily because mitochondrial dysfunctions often result in serious disorders such as myopathies or other neuro-degenerative diseases in human [1]. Mitochondria are frequently referred to as semi-autonomous because they contain genomes as well as comprehensive gene expression machineries. Contemporary mt-genomes still encode a relatively well-conserved core set of genes It is composed of two major classes of genes encoding key components required for energy production such as subunits of the respiratory chain complexes and factors required for mt-translation, in particular tRNAs. While mt-genome structures greatly vary, gene content is not correlated with the disparity of genome sizes. Beyond the universal conservation of core mt-functions of prokaryote origin, it is remarkable that mt-gene expression relies on a wide and diverse array of specific processes that have arisen during eukaryote evolution [13,14]. These processes involving a number of recently recognized factors are reviewed here together with the import pathways required to reach the full set of tRNAs in mitochondria as well as the original features that define the structure and function of mt-tRNAs
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