Two in vitro protein-synthesizing systems derived from E. coli have been utilized to demonstrate that the concentration of a tRNA species can regulate the rate of translation of a messenger RNA. (a) The rate of poly-U-directed C(14)-phenylalanine incorporation into protein is stimulated by concentrations of tRNA(Phe) from 1.5 x 10(-8) M to 3.0 x 10(-6) M, the latter representing a tRNA(Phe)/70S ribosome ratio of 7. (b) The rate of translation of poly A,G in a S-30 protein-synthesizing system derived from E. coli is limited by the amount of tRNA(Arg) recognizing the codewords AGA and AGG present in the extract. Polypeptide synthesis can be stimulated in direct proportion to the amount of this tRNA(Arg) species added to the reaction mixture. A mechanism for regulating the rate of protein synthesis at the translational level may be the slowing of polypeptide chain propagation at certain codons due to the presence of rate-limiting tRNA species.
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