Peptide Chain Initiation in Homologous and Heterologous Systems From Mitochondria and Bacteria.

Native ribosomal subunits from mitochondria of Neurospora crassa bind fMet‐tRNA and synthesize fMet‐puromycin at 5 mM Mg 2+. The reactions are stimulated by the trinucleotide ApUpG and by GTP. After washing the subunits with 1 M NH4CI the binding capacity is lost but can be restored by adding initiation factors from Escherichia coli Unwashed mitochondrial 73 S ribosomes require E. coli initiation factors for binding fMet‐tRNA and forming fMet‐puromycin. Cytoplasmic 77 S ribosomes from Neurospora are inactive in these chain initiation reactions, even in the presence of bacterial initiation factors. The requirement of the initiation reactions for AUG, GTP and initiation factors is similar in systems from E. coli, Bacillus subtilis and Neurospora mitochondria. Ribosomes and initiation factors from E. coli and B. subtilis are completely interchangeable. The results suggest that mitochondria from Neurospora use a bacteria‐like chain initiation mechanism.