Evolution of the tRNATyr/TyrRS aminoacylation systems

The tRNA identity rules ensuring fidelity of translation are globally conserved throughout evolution except for tyrosyl-tRNA synthetases (TyrRSs) that display species-specific tRNA recognition. This discrimination originates from the presence of a conserved identity pair, G1-C72, located at the top of the acceptor stem of tRNATyr from eubacteria that is invariably replaced by an unusual C1-G72 pair in archæal and eubacterial tRNATyr. In addition to the key role of pair 1-72 in tyrosylation, discriminator base A73, the anticodon triplet and the large variable region (present in eubacterial tRNATyr but not found in eukaryal tRNATyr) contribute to tyrosylation with variable strengths. Crystallographic structures of two tRNATyr/TyrRS complexes revealed different interaction modes in accordance with the phylum-specificity. Recent functional studies on the human mitochondrial tRNATyr/TyrRS system indicates strong deviations from the canonical tyrosylation rules. These differences are discussed in the light of the present knowledge on TyrRSs.