[Corrective steps in amino acid activation for protein biosynthesis].

Activation of aminoacids for proteinbiosynthesis is not absolutely specific. With tRNA modified at the 3' end we were able to show, that valine misactivated by isoleucyl-tRNA synthetase is transferred to tRNA(I1e). Val-tRNA(I1e) is then hydrolyzed by the enzyme prior to release of the wrong product. For this hydrolysis the nonaccepting 3'OH of the 3'-terminal ribose is essential. The role of the 3'OH is to activate a water molecule which, in the case of valine, is inserted into the place normally occupied by the methyl group of isoleucine. Hence hydrolysis is specific for misactivated valine while the correct substrate, isoleucine, is protected by its additional methyl group.