Import of tRNA into Yeast Mitochondria: Experimental Approaches and Possible Applications

Abstract

Mitochondria contain hundreds of polypeptides, only a minor part of which are encoded in the mitochondrial DNA and synthesized in the organelle. The majority of mitochondrial proteins are coded for by nuclear genes and are addressed to the organelle in the form of precursors from the cytoplasm. The mechanism of pre-protein import has been studied in detail, and a number of components of targeting and translocation across the mitochondrial membranes have been identified (Kübrich et al. 1995). Proteins are not the only type of macromolecules to be mitochondrially imported. In fact, several RNAs of nucleo-cytoplasmic origin were identified as associated with mitochondria: 5S rRNA, the RNA component of MRP RNase, RNase P RNA and numerous tRNAs (Schneider 1994). tRNA import can be considered as the best documented. Mitochondrially imported tRNAs were found in plants, yeast and protozoans (Table 1). The number of imported tRNAs varies among organisms and ranges between a single in the yeast S. cerevisiae and the totality of mitochondrial tRNAs in trypanosomatids. In some protozoans and in plants, the number of imported tRNAs is intermediate and species-specific (see Chap. 23, this Vol.). Studies of import mechanisms are very recent and rely on the development of model systems in vivo for the trypanosomes (Hauser and Schneider 1995; Schneider 1996; Lima and Simpson 1996), Tetrahymena (Rusconi and Cech 1996a,b), plants (Small et al. 1992) and yeast (Entelis et al. 1998) and, on the other hand, in vitro, for the trypanosomatids (Mahapatra et al. 1994; Adhya et al. 1997) and yeast (reviewed in: Tarassov and Martin 1996).