Mammalian Mitochondrial Ribosomes. Studies on the Exchangeability of Polypeptide Chain Elongation Factors from Bacterial and Mitochondrial Systems

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Abstract
Mammalian mitochondrial ribosomes from rat liver synthesized poly-Phe from [14C]-Phe-tRNA in the presence of a homologous 105 glalv supernatant fraction. The activity depended on the addition of synthetic template and was resistant to cycloheximide. The polyanion spermidine had a stimulatory effect on peptide synthesis in vitro. Escherichia coli ribosomes functioned with heterologous supernatant fraction; 55-S mitochondrial ribosomes were inactive when supplemented with heterologous supernatant fractions from E. coli or with purified bacterial elongation factors [EF]. EF-T slightly stimulated poly-Phe synthesis when added in combination with mitochondrial supernatant fractions. Two-dimensional electrophoretic analysis of the protein content of both supernatant fractions revealed considerable differences in the distribution of the species-specific proteins according to their isoelectric points. The mitochondrial supernatant proteins were in general more basic, and the few acidic proteins did not co-migrate with EF-Tu or EF-G from E. coli.