Three-dimensional models of the tRNA-like 3' termini of some plant viral RNAs.

Various plant viral RNAs possess a 3' terminus with tRNA-like properties. These viral RNAs are charged with an amino acid upon incubation with the cognate aminoacyl-tRNA synthetase and ATP. We have studied the structure of end-labelled 3'-terminal fragments of turnip yellow mosaic virus RNA and brome mosaic virus RNA 2 with chemical modifications of the adenosine and cytidine residues and with enzymatic digestions using RNase T1, nuclease S1 and the double-strand-specific ribonuclease from cobra venom. The data indicate that the 3' termini of these plant viral RNAs lack a cloverleaf structure as found in classical tRNA. The three-dimensional folding, however, reveals a striking resemblance with classical tRNA. The models proposed are supported by phylogenetic data. Apparently distinct three-dimensional solutions have evolved to meet the requirements for faithful recognition by tRNA-specific enzymes. The way in which the aminoacyl acceptor arms of these tRNA-like structures are constructed reveal novel features in RNA folding which may have a bearing on the secondary and tertiary structures of RNA in general. The dynamic behaviour of brome mosaic virus RNA 2 in solution presumably is illustrative of conformational transitions, which RNAs generally undergo on changing the ionic conditions.