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Human tyrosyl-tRNA synthetase from mitochondria (mt-TyrRS) presents dual sequence features characteristic of eubacterial and archaeal TyrRSs, especially in the region containing amino acids recognizing the N1-N72 tyrosine identity pair. This would imply that human mt-TyrRS has lost the capacity to discriminate between the G1-C72 pair typical of eubacterial and mitochondrial tRNATyr and the reverse pair C1-G72 present in archaeal and eukaryal tRNATyr. This expectation was verified by a functional analysis of wild-type or mutated tRNATyr molecules, showing that mt-TyrRS aminoacylates with similar catalytic efficiency its cognate tRNATyr with G1-C72 and its mutated version with C1-G72. This provides the first example of a TyrRS lacking specificity toward N1-N72 and thus of a TyrRS disobeying the identity rules. Sequence comparisons of mt-TyrRSs across phylogeny suggest that the functional behavior of the human mt-TyrRS is conserved among all vertebrate mt-TyrRSs.

The life cycle of Plasmodium falciparum, the agent responsible for malaria, depends on both cytosolic and apicoplast translation fidelity. Apicoplast aminoacyl-tRNA synthetases (aaRS) are bacterial-like enzymes devoted to organellar tRNA aminoacylation. They are all encoded by the nuclear genome and are translocated into the apicoplast only after cytosolic biosynthesis. Apicoplast aaRSs contain numerous idiosyncratic sequence insertions: An understanding of the roles of these insertions has remained elusive and they hinder efforts to heterologously overexpress these proteins. Moreover, the A/T rich content of the Plasmodium genome leads to A/U rich apicoplast tRNA substrates that display structural plasticity. Here, we focus on the P. falciparum apicoplast tyrosyl-tRNA synthetase (Pf-apiTyrRS) and its cognate tRNATyr substrate (Pf-apitRNATyr). Cloning and expression strategies used to obtain an active and functional recombinant Pf-apiTyrRS are reported. Functional analyses established that only three weak identity elements in the apitRNATyr promote specific recognition by the cognate Pf-apiTyrRS and that positive identity elements usually found in the tRNATyr acceptor stem are excluded from this set. This finding brings to light an unusual behavior for a tRNATyr aminoacylation system and suggests that Pf-apiTyrRS uses primarily negative recognition elements to direct tyrosylation specificity. [ABSTRACT FROM AUTHOR]

Examines the tyrosine identity determinants of Methanococcus (M.) jannaschii and Saccharomyces cerevisiae transfer (t) RNA. Tyrosylation properties of M. jannaschii and yeast tRNA; Correlation between archeal and eukaryotic aminoacylation systems; Determinants of tyrosine identity.

Abstract: 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. [Copyright &y& Elsevier]