Strategy for RNA recognition by yeast histidyl-tRNA synthetase.

Histidine aminoacylation systems are of interest because of the structural diversity of the RNA substrates recognized by histidyl-tRNA synthetases. Among tRNAs participating in protein synthesis, those specific for histidine all share an additional residue at their 5'-extremities. On the other hand, tRNA-like domains at the 3'--termini of some plant viruses can also be charged by histidyl-tRNA synthetases, although they are not actors in protein synthesis. This is the case for the RNAs from tobacco mosaic virus and its satellite virus but also those of turnip yellow and brome mosaic viruses. All these RNAs have intricate foldings at their 3'-termini differing from that of canonical tRNAs and share a pseudoknotted domain which is the prerequisite for their folding into structures mimicking the overall L-shape of tRNAs. This paper gives an overview on tRNA identity and rationalizes the apparently contradictory structural and aminoacylation features of histidine-specific tRNAs and tRNA-like structures. The discussion mainly relies on histidylation data obtained with the yeast synthetase, but the conclusions are of a more universal nature. In canonical tRNA(His), the major histidine identity element is the 'minus' 1 residue, since its removal impairs histidylation and conversely its addition to a non-cognate tRNA(Asp) confers histidine identity to the transplanted molecule. Optimal expression of histidine identity depends on the chemical nature of the -1 residue and is further increased and/or modulated by the discriminator base N73 and by residues in the anticodon. In the viral tRNA-like domains, the major identity determinant -1 is mimicked by a residue from the single-stranded L1 regions of the different pseudoknots. The consequences of this mimicry for the function of minimalist RNAs derived from tRNA-like domains are discussed. The characteristics of the histidine systems illustrate well the view that the core of the amino acid accepting RNAs is a scaffold that allows proper presentation of identity nucleotides to their amino acid identity counterparts in the synthetase and that different types of scaffoldings are possible.