MITOCHONDRIAL RIBOSOMAL-PROTEINS (MRPS) OF YEAST

Mitochondrial ribosomal proteins (MRPs) are the counterparts in that o rganelle of the cytoplasmic ribosomal proteins in the host. Although t he MRPs fulfil similar functions in protein biosynthesis, they are dis tinct in number, features and primary structures from the latter. Most progress in the eludication of the properties of individual MRPs, and in the characterization of the corresponding genes, has been made in baker's yeast (Saccharomyces cerevisiae). To date, 50 different MRPs h ave been determined, although biochemical data and mutational analysis propose a total number which is substantially higher. Surprisingly, o nly a minority of the MRPs that have been characterized show significa nt sequence similarities to known ribosomal proteins from other source s, thus limiting the deduction of their functions by simple comparison of amino acid sequences. Further, individual MRPs have been character ized functionally by mutational studies, and the regulation of express ion of MRP genes has been described. The interaction of the mitochondr ial ribosomes with transcription factors specific for individual mitoc hondrial mRNAs, and the communication between mitochondria and the nuc leus for the co-ordinated expression of ribosomal constituents, are ot her aspects of current MRP research. Although the mitochondrial transl ational system is still far from being described completely, the yeast MRP system serves as a model for other organisms, including that of h umans.