Structural and Functional tRNA Mimicry of the 3’-end of Turnip Yellow Mosaic Virus RNA
The 3’-ends of several plant viral RNAs show a number of functional characteristics of tRNAs1,2; they are recognized by a set of tRNA-specific proteins, including aminoacyl-tRNA synthetases. So, the RNA of turnip yellow mosaic virus can be valylated by yeast valyl-tRNA synthetase3,4 with kinetic constants very close to those of the aminoacylation of yeast tRNAVal by this enzyme5. The 3′-ends of these viral RNAs however, are lacking several characteristic primary structural features of tRNAs such as strategic D- or T-loop sequences and modified bases. Moreover, they cannot be folded a priori into a canonical tRNA cloverleaf. In the case of TYMV RNA, the question arose as to “how do two structures as different as tRNAVal and the 3′-region of the viral RNA behave in such a similar fashion in the presence of valyl-tRNA synthetase?”. This question might be answered if one supposes that similar structural domains are recognized by the synthetase and exist at the level of the three dimensional structures and even at the level of their secondary structures. In this view, the secondary structure of the 3′-end of TYMV RNA was established by enzymatic footprinting methodologies6. A three-dimensional L-shaped conformation mimicking tRNA, but involving a new RNA folding principle, the pseudoknot, was proposed by the Leiden group7,8 for the 86 last nucleotides of this RNA. A rigourous graphical modelling allowed to assess the reality of this pseudoknotted folding9. To understand the involvement of the 3′-end of the TYMV RNA in aminoacylation, the direct contact points between this part of the RNA and yeast valyl-tRNA synthetase were determined10. Moreover, the study of the valylation of tRNA-like transcripts from cloned cDNA of TYMV RNA permitted to determine the minimal length of the RNA necessary for optimal valylation11.
KeywordsBrome Mosaic Virus Turnip Yellow Mosaic Virus Plant Viral RNAs Anticodon Stem Viral Fragment
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