Import of RNAs into Plant Mitochondria

Abstract

The plant mitochondrial genetic system has to provide about 35 ­polypeptides, which are essential for cell survival as they are components of the respiratory chain or contribute to its biogenesis. Thus, an active mitochondrial translation system is an absolute requisite. With the exception of a few ribosomal proteins, all protein factors involved in plant mitochondrial translation are nuclear-encoded and imported via the classical protein import channel. At the RNA level, the three ribosomal RNAs are encoded by the mitochondrial genome. In contrast, the transfer RNA (tRNA) population encoded by the plant mitochondrial genome is not sufficient to decode the 61 sense codons of the universal genetic code used by plant mitochondria. It is now well established that to compensate for the lack of tRNAs, several nuclear-encoded tRNAs used by the cytosolic translation machi-nery are also found in the mitochondrion. In this review, evolutionary aspects and functions of imported tRNAs are presented. Then, the basic questions on the tRNA mitochondrial import selectivity, regulation, targeting, and translocation in plants are discussed and compared to what has been discovered in tRNA mitochondrial import in evolutionary divergent organisms.

Glossary

Aminoacyl-tRNA synthetase:

enzyme that catalyzes the amino acid attachment at the 3′ extremity of a tRNA molecule.

Angiosperm:

the flowering plants.

Bryophyte:

embryophytes (often called “land plants”) that are nonvascular. They neither have flowers nor produce seeds and reproduce via spores.

Cytochromec:

a small heme (an iron atom contained in a large heterocyclic ring called porphyrin) protein associated with the inner mitochondrial membrane that is an essential component for the electron transport chain.

D-arm:

in the secondary structure of a “classical” tRNA molecule, the D-arm is usually a 4 bp stem ending in a loop that often contains dihydrouridine.

Enolase:

a metalloenzyme of the glycolysis that converts the 2-phosphoglycerate to phosphoenolpyruvate.

Gymnosperm:

a group of spermatophyte seed-bearing plants with ovules on scales of a cone or similar structure.

Lysidine:

a derivative of cytidine in which the carbonyl is replaced by the amino acid lysine. Lysidine typically occurs in the anticodon of a tRNA molecule.

Oxidative phosphorylation:

in mitochondria, a metabolic pathway that uses energy released by the oxidation of NADH and succinate to produce ATP.

Selenocysteine:

an amino acid. It has a structure similar to cysteine, but with an atom of selenium taking the place of the sulfur. Often abbreviated Sec.

Translocase of the outer mitochondrial membrane:

a protein complex located in the outer mitochondrial membrane of mitochondria and involved in the translocation of nuclear-encoded proteins from the cytosol to the inter membrane space of mitochondria.