Function of Met‐tRNAMetf and Met‐tRNAMet in Peptide‐Chain Elongation in Cell‐Free Systems from Mouse‐Liver and Ascites‐Tumor Cells

The two methionine‐accepting tRNA species from mouse ascites tumor cells were investigated with respect to their function in peptide chain elongation, using a mouse liver polysome system and a T‐factor dependent ribosomal‐binding assay. In support of results published previously it was found that in a non‐initiating cell‐free system Met‐tRNAMetf inserts methionine into internal positions of nascent peptide chains, even in the presence of Met‐tRNAMet. Correspondingly, T factor from ascites tumor cells stimulated the binding of both methionine‐accepting tRNAs to washed ribosomes. When Met‐tRNAMet and Met‐tRNAMetf were introduced together into the ribosomal binding assay, a strong preference for the T‐factor‐induced attachment of Met‐tRNAMet became visible. Formylated initiator tRNA was not recognized by T factor Whereas the ribosomal binding of formylated or unformylated initiator tRNA promoted by initiation factors functioned equally well with poly(A,U,G) and the triplet ApUpG, the T‐factor‐induced attachment of both Met‐tRNAMet species required the presence of the polynucleotide. Competition experiments with high concentration of Val‐tRNAVal and Met‐tRNAMetf revealed the occurrence of a partial mutual interference of these two tRNA species during chain elongation. The magnitude of this effect, however, was not sufficient to attribute the function of Met‐tRNAMetf in chain elongation to miscoding. No competition was observed between Val‐tRNAVal and Met‐tRNAMet.