Protein Synthesis in Rabbit Reticulocytes: FACTORS CONTROLLING INTERNAL AND TERMINAL METHIONINE CODON RECOGNITION BY THE METHIONYL TRANSFER RIBONUCLEIC ACID SPECIES

Abstract

Abstract In rabbit reticulocyte protein synthesis, Met-tRNAfmet (liver) can recognize the terminal AUG and GUG codons. The preferential recognition of the terminal methionine codons (AUG and GUG) by Met-tRNAfmet (liver) requires the presence of a peptide chain initiation factor or factors and low Mg++. Both Met-tRNAfmet and Met-tRNAmmet can recognize internal AUG codons, although the affinity of Met-tRNAmmet for the same codons is greater than that of Met-tRNAfmet. When both Met-tRNAmet species are present in excess, Met-tRNAmmet preferentially transfers methionine into polypeptides in response to internal AUG codons. Met-tRNAmmet and not Met-tRNAfmet can recognize internal GUG codons. However, this recognition of internal GUG codons by Met-tRNAmmet (liver) requires a higher Mg++ concentration (9 mm). Yeast Met-tRNAfmet, like the liver Met-tRNAfmet, can also recognize the terminal AUG and GUG codons and can transfer methionine into the terminal positions of the peptide chains in the presence of the peptide chain initiation factors at low Mg++. However, yeast Met-tRNAfmet does not efficiently recognize the internal AUG codons. On the other hand, Escherichia coli Met-tRNAfmet transfers methionine very efficiently in response to internal AUG codons, although the transfer efficiency of methionine into polypeptides by this species in response to terminal AUG and GUG codons is comparatively low.