Roles of methionine transfer RNA's in protein synthesis in rabbit reticulocytes

Abstract

DEAE-Sephadex column chromatography of crude rabbit liver transfer RNA gives rise to three peaks of methionine-acceptor activity. Only one of these tRNA Met species (tRNA f Met) † † The abbreviations for the polynucleotides used in this study have been described previously ( Gupta, 1968; Wells et al., 1967 ) but have been modified to conform to IUPAC conventions. Methionine sulfone being abbreviated to Met-SO 2: tRNA Met, uncharged methionyl-tRNA; tRNA f Met, uncharged methionyl-tRNA species which can be formylated; tRNA m Met, uncharged methionyl-tRNA species which cannot be formylated; Met-tRNA f Met, charged methionyl-tRNA species which can be formylated; Met-tRNA m Met, charged methionyl-tRNA species which cannot be formylated could be charged by Escherichia coli synthetase and, as previously reported, could be formylated by E. coli formylase. The other two tRNA Met species (tRNA m1 Met and tRNA m2 Met) could not be charged by E. coli synthetase. The coding properties of the tRNA Met species were studied in a cell-free protein synthesizing system obtained from rabbit reticulocytes with poly r(A-U-G) messenger and endogenous messenger in reticulocyte ribosomes. These studies indicated that with rabbit reticulocytes, tRNA f Met recognizes specifically the terminal methionine codon (AUG) and transfers methionine to the N-terminal positions of the polypeptide chains while tRNA m Met recognizes specifically the internal methionine codon (AUG) and inserts methionine into the internal positions of the polypeptide chains.