The cellular power plants of eukaryotic cells, mitochondria have their own genome, separate from the one in the cell's nucleus. This particularity can be explained by their origin. They actually correspond to the remnants of bacteria that entered a primitive eukaryotic cell approximately two billion years ago. As evolution took place, the mitochondrial genome weakened, losing genes or transferring part of them to the nucleus of the host cell. Therefore, the mitochondria needed a hand to help them replicate, transcribe and translate their genome. Subsequently, they imported proteins from the cell’s cytosol. Recently, scientists discovered that transfer RNAs were imported into the mitochondria. Part of the research of Claire Remacle, head of the University of Liège’s laboratory of genetics of microorganisms, aims to elucidate the mechanisms that regulate this phenomenon. 

Mitochondria are the central power plant of eukaryotic cells. It is within these organelles that the energy provided by the organic molecules is recuperated in the form of ATP, the cell’s fuel. Even though the mitochondria are a component of the eukaryotic cell and, as such, respond to "orders" given by the DNA from the nucleus, they have their own DNA which ensures they function correctly. This particularity can be explained by going back to the origins of these organelles.  It is now common knowledge that they correspond to the remnants of bacteria that entered a primitive eukaryotic cell approximately two billion years ago by endosymbiosis. As evolution took place, the genome of these endosymbionts weakened, losing genes or transferring part of them to the nucleus of the host cell. Thus, little by little, these endosymbionts have gone from being autonomous organisms to the state of semi-autonomous organelles.
Mitochondrial DNA is tiny. "In man, for instance, it is approximately 16  kilobases (kb) compared with three million kilobases  for the nuclear genome", explains Claire Remacle fellow at the Faculty of Sciences and in charge of the University of Liège’s laboratory of genetics of microorganisms.

tRNA, an intermediary between mRNA and amino acid

Like its big brother, the mitochondrial genome replicates and is transcribed and translated into proteins. However, it cannot take on the production of the proteins necessary for the functioning of the mitochondria alone. “We have known for a long time that proteins are imported into the mitochondria from the cell’s cytosol”, Claire Remacle points out. “But the importing of transfer RNAs into the mitochondria is a more recent discovery. This was first observed in protozoa, then in higher plants and finally in mammals”, the scientist continues.

The transfer RNAs or tRNAs act rather like adaptors during the process of translating messenger RNAs (mRNA) into proteins. The succession of amino acids that compose a protein appear as code in the mRNA, in the form of a succession of codons. “There are 63 codons for some 20 amino acids, which means that each amino acid can be represented by several codons. Some of them have up to six corresponding codons”, Claire Remacle explains. Only the tRNAs are capable of reading these codons thanks to a group of three nucleotides located in their structure and known as anticodon. The latter specifically pairs up with the codon's complementary sequence present on the mRNA. Besides this anticodon, the tRNA carries the corresponding amino acid to the intended fixation site. The “meeting” between the tRNA and the codons lodged on the mRNA takes place in the ribosomes, where the synthesis of the link between the amino acids comprising the protein occurs (see diagram).