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

Mammalian mitochondrial ribosomes from rat liver synthesised poly(phenyla1anine) from [‘“CIPhe-tRNA in the presence of a homologous lo5 x g,, supernatent 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. In contrast to Escherichiu coli ribosomes, which also functioned with heterologous supernatant fractions, 55-S mitochondrial ribosomes were inactive when supplemented with heterologous supernatant fractions from E. coli or with purified bacterial elongation factors. EF-T slightly stimulated polyphenylalanine 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 superand the few acidic proteins did not co-migrate with natant proteins were in general more basic EF-Tu or EF-G from E. coli.