How widespread is mitochondrial tRNA import?
Bioinformatic analyses may not reveal the true occurrence of mitochondrial tRNA import for two reasons:

• Modifications of the wobble nucleotide cannot be predicted from the sequence of the mitochondrial genomes. However, they often change the decoding capacity of a tRNA. It is therefore not always possible to predict the number of tRNAs that are required to decode a specific set of codons.
• The existence of a mitochondrial tRNA gene does not preclude mitochondrial import of a cytosolic tRNA that is able to read the same codons. In fact, redundant tRNA import provides a scenario of how mitochondrial tRNA genes could have been lost during evolution.

The limitations to identify tRNA import by bioinformatic approaches are of practical importance. It has recently been shown that a few cytosolic tRNAs are imported into mitochondria of humans and of the yeast Saccharomyces cerevisiae, even though their mitochondrial genomes encode an apparently complete set of mitochondrial tRNAs. Furthermore, the same studies also suggested that yeast mitochondria might have two distinct pathways for tRNA import. These results show that import of tRNAs is much more widespread than predicted by bioinformatic methods and offer the exciting prospect that mitochondrial tRNA import might be universal.

References
Rubio, M.A., Rinehart, J.J., Krett, B., Duvezin-Caubet, S., Reichert, A.S., Söll, D., and Alfonzo, J.D. (2008)
Mammalian mitochondria have the innate ability to import tRNAs by a mechanism distinct from protein import
Proc. Natl. Acad. Sci. USA 9186-9191.

Rinehart, J., Krett, B., Rubio, M.A., Alfonzo, J.D., and Soll, D. (2005)
Saccharomyces cerevisiae imports the cytosolic pathway for Gln-tRNA synthesis into the mitochondrion
Genes Dev. 19, 583-592.