SELENOCYSTEINE TRANSFER-RNA AND SERINE TRANSFER-RNA 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]See) for aminoacylation correspond to elements for seryl-tRNA synthetase recognition. As tRNA([Ser]Sec) has low homology to the tRNA(Ser) 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 tRNA(Ser):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 tRNA(Ser) 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 tRNA(Ser), while the long extra arm plays an important role which seems to be orientation- and length-specific in tRNA(Ser) 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 T psi C stem contribute to the recogntion process, but to a lesser extent than the discriminator base and the long extra arm. (C) 1994 Academic Press, Inc.