Position of aminoacylation of individual Escherichia coli and yeast tRNAs.

Abstract
TRNAs terminating in 2''-or 3''-doexyadenosine were prepared from unfractionated E. coli and yeast (Saccharomyces cerevisiae) tRNAs and purified to remove unmodified tRNAs. The modified tRNA species were assayed for aminoacylate with each of the 20 amino acids to determine the initial position of tRNA aminoacylation. The E. coli and yeast aminoacyl-tRNA synthetases specific for arginine, isoleucine, leucine, methionine, phenylalanine, and valine, and the E. coli glutamyl-tRNA synthetase aminoacylated only those congnate tRNAs terminating in 3''-deoxydenosine (i.e., those having a 2''-OH group. Those E. coli and yeast synthetases specific for alanine, glycine, histidine, lysine, proline, serine, threonine, and the yeast synthetase specific for glutamine, utilized exclusively those tRNAs having an available 3''-OH group on the 3''-terminal nucleoside; the E. coli and yeast synthetases specific for asparagine, cysteine, and tyrosine, and the yeast aspartly-tRNA synthetase, utilized both of the modified cognate tRNAs. The only observed difference in specificity between the E. coli and yeast systems was for tRNAtrp, which was aminoacylated on the 2''-position in E. coli and the 3''-position in yeast. The initial position of aminoacylation is apparently not uniform for all tRNAs, although for individual tRNAs the specificity was conserved during the evolution from a prokaryotic to eukaryotic organism.

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