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.