Participation of redundant transfer ribonucleic acids from yeast in protein synthesis

Four glycine tRNAs have been isolated from brewers yeast. They appear to recognize the same code word but are known to be structurally different. In the present experiments, each glycine tRNA has been pre-charged with radioactive glycine and has been used in the in vitro synthesis of protein directed by bacteriophage R17 RNA in an Escherichia coli S-30 system that was dependent on added transfer ribonucleic acid (tRNA) for maximal activity. Preliminary experiments established that the yeast glycyl-tRNAs transferred their amino acid into the same proteins that were synthesized when E. coli tRNA was used. Large-scale preparations fractionated on sucrose gradients allowed the isolation of the coat protein subunits synthesized in vitro. Thin-layer mapping of tryptic digests of protein synthesized in vitro has shown that for the most part, the glycine code words are translated with considerable fidelity by the yeast glycyl-tRNAs. The yeast tRNAs show a characteristic pattern of competition between themselves, and also when compared to E. coli tRNA^Gly when used in pairs for the synthesis of coat protein. One of the peptides in the R17 coat has more glycine incorporated into it from yeast tRNA^Gly than would be expected from the known composition of the peptide. It appears likely that a code word for another amino acid in the R17 RNA is being translated as glycine by the yeast, but not the E. coli, tRNAs. These observations have formal similarities to reports of suppression of missense being brought about by tRNA.