The biogenesis of mitochondria 29. Effects of temperature-induced phase changes in membranes on protein synthesis by mitochondria

Mitochondrial ribosomal proteins (MRPs) of yeast

Mitochondrial ribosomal proteins (MRPs) are the counterparts in that organelle of the cytoplasmic ribosomal proteins in the host. Although the MRPs fulfil similar functions in protein biosynthesis, they are distinct 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 have been determined, although biochemical data and mutational analysis propose a total number which is substantially higher. Surprisingly, only a minority of the MRPs that have been characterized show significant sequence similarities to known ribosomal proteins from other sources, thus limiting the deduction of their functions by simple comparison of amino acid sequences. Further, individual MRPs have been characterized functionally by mutational studies, and the regulation of expression of MRP genes has been described. The interaction of the mitochondrial ribosomes with transcription factors specific for individual mitochondrial mRNAs, and the communication between mitochondria and the nucleus for the co-ordinated expression of ribosomal constituents, are other aspects of current MRP research. Although the mitochondrial translational system is still far from being described completely, the yeast MRP system serves as a model for other organisms, including that of humans.

Are all mitochondrial translation products synthesized on membrane-bound ribosomes?

The Arrhenius kinetics for the synthesis of var1, which is a hydrophilic protein of the mitochondrial ribosomes, have been compared to that of other mitochondrial translation products, which are hydrophobic subunits of the respiratory enzyme complexes. Our results indicate that, in the yeast Saccharomyces cerevisiae, the hydrophilic var1 protein is synthesized on membrane-associated mitochondrial ribosomes which cannot be distinguished from those responsible for the synthesis of the hydrophobic mitochondrial translation products.

Effect of temperature on protein synthesis and leucine transport by yeast mitochondria

Protein synthesis in yeast mitochondria shows biphasic Arrhenius plots both in vivo and in vitro, with a twofold increase in the activation energy below the transition temperature suggesting a functional association between mitochondrial protein synthesis and the inner membrane. Analysis by gel electrophoresis of mitochondrial translation products labeled in vivo showed that the same proteins are synthesized and then inserted into the membrane above and below the transition temperature of the membrane. The rate of leucine uptake into mitochondria was decreased at least fivefold in the presence of chloramphenicol, suggesting that leucine is used mainly for protein synthesis. In the absence of chloramphenicol, the rate of leucine uptake was always slightly higher but comparable to the incorporation rate of leucine into protein at all temperatures studied, suggesting that the transport of leucine into mitochondria is not rate-limiting for protein synthesis. The ionophore valinomycin or the uncoupler carbonyl phenylhydrazone (CCCP) inhibited 75-80% of the leucine uptake in the presence of chloramphenicol. In addition, the omission of respiratory chain substrates and the ATP-regenerating system led to a 93% inhibition of uptake, suggesting that leucine uptake may occur by an active transport mechanism.

Effect of transfer ribonucleic acid dimer formation on polyphenylalanine biosynthesis

Escherichia coli tRNAPhe (anticodon GAA) as well as yeast tRNAPhe (anticodon GmAA) forms a strong complex with E. coli tRNAGlu (anticodon s2UUC) through an interaction between their complementary anticodons. This interaction inhibits aminocylation of tRNAPhe but not the formation of a complex with elongation factor Tu. Moreover, at 0 degrees C, tRNAGlu strongly inhibits the binding of Phe-tRNA to poly(U)-programmed ribosomes via either the enzymic (EF-Tu-promoted) or nonenzymic pathway. At 15 degrees C, tRNAGlu effectively inhibits polyphenylanine synthesis in the E. coli system. The inhibition is reversed at 37 degrees C, where the Phe-tRNA.tRNAGlu dimer is dissociated. Calculations based upon the E. coli intracellular concentrations of tRNAs and the published rates of association and dissociation of the tRNA dimers suggest that this interaction may inhibit protein synthesis in vivo at temperatures below 15 degrees C.

Biogenesis of mitochondria 36, The genetic and biochemical analysis of a mitochondrially determined cold sensitive oligomycin resistant mutant of Saccharomyces cerevisiae with affected mitochondrial ATPase assembly

The isolation and characterisation of a mutant affecting the assembly of mitochondrial ATPase is reported. The mutation confers resistance to oligomycin and venturicidin and sensitivity of growth on nonfermentable substrates to low temperature (19degrees). Genetic analysis indicates that the phenotype is due to a single mutation located on the mitochondrial DNA which is probably allelic with the independently isolated oligomycin resistance mutation [oli1-r]. Growth of the mutant at the non-restrictive temperature (28degrees) yields mitochondria in which the ATPase appears more sensitive to oligomycin than that of the sensitive parental strain. However, when the enzyme is isolated free from the influence of the membrane strong resistance to oligomycin is evident. These data suggest that the component responsible for the oligomycin resistance of the ATPase is part of or subject to interaction with the mitochondrial inner membrane. Measurements of the ATPase content of mitochondria indicate that ATPase production is impaired during growth at 19degreesC. In addition, studies of the maximum inhibition of mitochondrial ATPase activity by high concentrations of oligomycin suggest a selective lesion in ATPase assembly at low temperature. The nett result is that during growth at 19degrees only about 10% of the normal level of ATPase is produced of which less than half is membrane integrated and thus capable of oxidative energy production. We propose that the mutation affects a mitochondrially synthesised membrane sector peptide of the ATPase which defines the interaction of F1ATPase with specific environments on the mitochondrial inner membrane.