| Abstract: | We have reciprocally transplanted the anticodon arm sequences of a set of amber suppressor tRNA genes, using recombinant DNA techniques. By this means, a very efficient suppressor may be converted to a poor one, and the poorest tRNA to the efficiency of the best one. In tRNA molecules of normal 2 degrees and 3 degrees structure, the suppressor efficiencies of different composite tRNAs having the same anticodon arm sequence are approximately the same. Large numbers of simultaneous changes throughout the rest of the molecule do not affect the efficiency. Selective nucleotide modification as a result of varied anticodon arm sequences cannot explain these efficiencies. Efficiencies are also unlikely to differ because of selective aminoacylation. Measurement of in vivo tRNA shows, however, that tRNA levels do vary if the anticodon arm sequence is changed. If tRNA levels are normalized, the anticodon arm effect on the translational efficiency remains. Therefore, different anticodon arms, all of normal secondary structure, are not equivalent in translation. The most efficient sequences in this series resemble those found in natural tRNAs associated with similar anticodons, as is proposed in the extended anticodon theory (Yarus, M. (1982) Science 218, 646-652). These molecules also provide some information on the specificity of nucleotide modification enzymes and on determinants of the steady-state tRNA level. |
