Glu-Q-tRNA synthetase coded by the gene, a new paralog of aminoacyl-tRNA synthetase that glutamylates tRNA anticodon

Analysis of the completed genome sequences revealed presence in various bacteria of an open reading frame (ORF) encoding a polypeptide chain presenting important similarities with the catalytic domain of glutamyl-tRNA synthetases but deprived of the C-terminal anticodon-binding domain. This paralog of glutamyl-tRNA synthetases, the YadB protein, activates glutamate in the absence of tRNA and transfers the activated glutamate not on tRNAGlu but instead on tRNAAsp. It has been shown that tRNAAsp is able to accept two amino acids: aspartate charged by aspartyl-tRNA synthetase and glutamate charged by YadB. The functional properties of YadB contrast with those of the canonical glutamyl-tRNA synthetases, which activate Glu only in presence of the cognate tRNA before aminoacylation of the 3′-end of tRNA. Biochemical approaches and mass spectrometry investigations revealed that YadB transfers the activated glutamate on the cyclopenthene-diol ring of the modified nucleoside queuosine posttranscriptionally inserted at the wobble position of the anticodon-loop to form glutamyl-queuosine. Unstability of the ester bond between the glutamate residue and the cyclopenthene-diol (half-life 7.5 min) explains why until now this modification escaped detection. Among Escherichia coli tRNAs containing queuosine in the wobble position, only tRNAAsp is substrate of YadB. Sequence comparison reveals a structural mimicry between the anticodon-stem and loop of tRNAAsp and the amino acid acceptor-stem of tRNAGlu. YadB, renamed glutamyl-Q-tRNAAsp synthetase, constitutes the first enzyme structurally related to aminoacyl-tRNA synthetases which catalyzes a hypermodification in tRNA, and whose function seems to be conserved among prokaryotes. The discovery of glutamyl-Q-tRNAAsp synthetase breaks down the current paradigm according to which the catalytic domain of aminoacyl-tRNA synthetases recognizes the amino acid acceptor-stem of tRNA and aminoacylates the 3′-terminal ribose. The evolutionary significance of the existence of an aminoacyl-tRNA synthetase paralog dedicated to the hypermodification of a tRNA anticodon will be discussed.