The Contrasting Role of strA and ram Gene Products in Ribosomal Functioning

The central act in translation of messenger RNA (mRNA) is the selection of a transfer RNA (tRNA) molecule specifically satisfying the con-ditions imposed by each codon present in the messenger script. It is well known that the deter-mining factor is the codon-anticodon binding by base-pairing, but it is now an accepted notion that the ribosome also plays a role in this selection. This was realized when it was found that informational suppression could be brought about pheno-typically by external factors like streptomycin, and that ribosomal mutations may interfere with this suppression (Gorini and Kataja, 1964). Attention was focused on a new aspect of ribo-somal function: fidelity in code translation. We are interested in this aspect of the ribosome role and in the ribosome mutations which define it. THE INTRINSIC AMBIGUITY By introducing alterations in a conventional (Nirenberg and Matthaei, 1961) amino acid incorporating system, one may inquire how code deciphering may go wrong. Table 1 shows the result of such an experiment in which the trans-lation tools are derived from Escherichia coli except for the messenger, synthetic polyuridylate, which is known to code for phenylalanine. The misincorporation f isoleucine under three different conditions is given in per cent of the total iso-leucine available which is the same in all conditions tested. It is seen, as expected, that under standard conditions (column 2) the misincorporation is minimal, but that it occurs in substantial mounts when these conditions are altered. Thus iso-leucine is incorporated along with phenylalanine when streptomycin is added (8/~g/ml) to the system (column 3). This is not surprising since it is known that isoleucine is one of the amino acids for