Structure of an Unmodified tRNA Molecule

We have used NMR to study the structure of the yeast tRNA^phe sequence which was synthesized by using T7 RNA polymerase. Many resonances in the imino ¹H spectrum of the transcript have been assigned, including those of several tertiary interactions. When the Mg²⁺ concentration is high, the transcript appears to fold normally, and the spectral features of the transcript resemble those of tRNA^phe. The transcript has been shown to be aminoacylated with kinetics similar to the modified tRNA^phe [Sampson, J. R., & Uhlenbeck, O. C.(1988) Proc, Natl. Acad. Sci. U.S.A. 85,1033-1037], suggesting that the structure of the two molecules must be similar. In the absence of Mg²⁺ or at [tRNA]:[Mg²⁺] ratios <0.2, the transcript does not adopt the native structure, as shown by both chemical shifts ana NOE patterns. In these low Mg²⁺ conditions, a second GU base pair is found, suggesting a structural rearrangement of the transcript. NMR data indicate that the structure of a mutant having G20 changed to U20 is nearly identical with that of the normal sequence, suggesting that the low aminoacylation activity of this variant is not due to a substantially different conformation.