The Import of Cytosolic tRNA into Plant Mitochondria

Abstract

A consideration of the sparse genetic information carried by mitochondrial genomes illustrates why these organelles are so dependent on the rest of the cell, as most of the proteins necessary for their function are encoded by the nuclear genome and imported from the cytosol. However, certain protein components of the respiratory chain complexes and (in some cases) a few ribosomal proteins are synthesized by the mitochondrial translation machinery. Hence, the mitochondrial translation system must include a full set of transfer RNAs (tRNAs) to ensure the incorporation of the usual 20 amino acids into the polypeptides being synthesized. Vascular plant mitochondrial genomes contain at most 12 to 14 functional “native” tRNA genes deriving from the genome of the original endosymbiotic bacterium (Sangaré et al. 1990; Weber-Lotfi et al. 1993; Ceci et al. 1996; Unseld et al. 1997). These genes are obviously insufficient to cover the needs of the mitochondrial translation system. To get round this problem, plant mitochondria use tRNAs that have to be imported from the cytosol (Dietrich et al. 1992; Kumar et al. 1996). This specific transport of nucleic acids across the double mitochondrial membrane occurs in many diverse organisms, including plants, some fungi and protozoa (Schneider 1994), and it can be expected that a sufficient understanding of the tRNA import mechanism will permit the development of a strategy for gene therapy of human cytopathies such as MELAS or MERRF provoked by mutations in mitochondrial tRNA genes.