Selenocysteine tRNA and Serine tRNA Are Aminoacylated by the Same Synthetase, but May Manifest Different Identities with Respect to the Long Extra Arm

Selenocysteine (Sec) tRNA[Ser]Sec donates Sec to protein, but interestingly, this amino acid is synthesized on tRNA which is first aminoacylated with serine. Thus, the identity elements in tRNA[Ser]Sec for aminoacylation correspond to elements for seryl-tRNA synthetase recognition. As tRNA[Ser]Sec has low homology to the tRNASer isoacceptors, it would seem then that the identity elements in tRNA[Ser]Sec involve (1) very specific sequences, (2) conformational features, and/or (3) different points or domains for tRNA[Ser]Sec:synthetase and tRNASer:synthetase recognition. Initially, we confirmed that the same synthetase aminoacylates both tRNAs by showing that a mutant tRNA[Ser]Sec which has a blocked 3′-terminus is a competitive inhibitor of tRNASer aminoacylation with a partially purified and a highly purified seryl-tRNA synthetase preparation. The discriminator base (base G73) is essential for aminoacylation of tRNA[Ser]Sec and tRNASer, while the long extra arm plays an important role which seems to be orientation- and length-specific in tRNASer and, in addition, may manifest sequence specificity in tRNA[Ser]Sec. This difference in the tRNA recognition specificity is discussed. The acceptor stem, DHU stem, and TψC stem contribute to the recogntion process, but to a lesser extent than the discriminator base and the long extra arm.