Eukaryotic selenocysteine tRNA has the 9/4 secondary structure

There are two secondary structure models for the eukaryotic selenocysteine (Sec) tRNA(Sec). One model, the 9/4 structure, was experimentally tested an d possesses acceptor and T-stems with 9 and 4 bp, respectively [Sturchler e t al., 1993; Hubert et al., 1998]. The other one, the 7/5 secondary structu re with a bulge in the T-stem, was derived from theoretical calculation [Io udovitch and Steinberg, 1999], In this report, we show more experimental re sults supporting the 9/4 secondary structure, Several tRNA(Sec) mutants, wh ose secondary structure can adopt only the 9/4 structure, were active for s erylation and selenylation, Some mutants that cannot base-pair between posi tions 26 and 44 to provide the 6 bp anticodon stem were still active, incon sistent with the model by Steinberg. We also show that the orientation of t he V-arm directly or indirectly influences the selenylation activity, and t hat the rigid 6 bp D-stem is important. Finally, we conclude that all tRNAs (Sec) possess the 13 bp domain II made by the stacking of the colinear AA a nd T-stems, whether they present the 9/4 structure in Eukarya and Archaea o r the 8/5 structure in bacteria. (C) 2000 Federation of European Biochemica l Societies.