Differential coupling efficiency of chemically activated amino acid to tRNA.

Interaction based on possible chemical affinity of an amino acid for tRNA was examined as a model for the aminoacylation of primitive tRNA without aid of an enzyme system. Two types of reaction were carried out and compared. One was the acyl linkage of amino acid to the 5'-terminal phosphate of a tRNA activated as an imidazolide. The other was the incorporation of an amino acid activated as an imidazolide into 2'(3')-hydroxyl groups of intact tRNA. Both types of reaction indicated that none of the amino acids tested had any selectivity for the tRNAs examined. However, the rates of reaction with a given tRNA were different among amino acids. In the second type of reaction, amino acids were found mainly at loop-out regions of tRNA, but not at either its 5'- or 3'-terminal sites.