Conservation of a tRNA core for aminoacylation

The core region of Escherichia coli tRNA(Cys)is important for aminoacylation of the tRNA. This core contains an unusual G15:G48 base pair, and three adenosine nucleotides A13, A22 and A46 that are likely to form a 46:[13:22] adenosine base triple. We recently observed that the 15:48 base pair and the proposed 46:[13:22] triple are structurally and functionally coupled to contribute to aminoacylation. Inspection of a database of tRNA sequences shows that these elements are only found in one other tRNA, the Haemophilus influenzae tRNA(Cys). Because of the complexity of the core, conservation of sequence does not mean conservation of function. We here tested whether the conserved elements in H. influenzae tRNA(Cys)were also important for aminoacylation of H. influenzae tRNA(Cys). We cloned and purified a recombinant H. influenzae cysteine-tRNA synthe-tase and showed that it depends on 15:48 and 13, 22 and 46 in a relationship analogous to that of E. coli cysteine-tRNA synthetase. The functional conservation of the tRNA core is correlated with sequence conservation between E.coli and H.influenzae cysteine-tRNA synthetases. As the genome of H. influenzae is one of the smallest and may approximate a small autonomous entity in the development of life, the dependence of this genome on G15:G48 and its coupling with the proposed A46:[A13:A22] triple for aminoacylation with cysteine suggests an early role of these motifs in the evolution of decoding genetic information.