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Arabidopsis tRNA Adenosine Deaminase Arginine Edits the Wobble Nucleotide of Chloroplast tRNA^Arg(ACG) and Is Essential for Efficient Chloroplast Translation^[W]

^a Australian Research Council Centre of Excellence in Plant Energy Biology, University of Western Australia, Crawley, 6008 WA, Australia
^b Institut de Biologie Moléculaire des Plantes du Centre National de la Recherche Scientifique, Université de Strasbourg, 67084 Strasbourg Cedex, France
^c Australian Research Council Centre of Excellence in Plant Energy Biology, School of Biology, Australian National University, Canberra, 0200 ACT, Australia

¹ Address correspondence to jose.gualberto{at}ibmp-ulp.u-strasbg.fr.

RNA editing changes the coding/decoding information relayed by transcripts via nucleotide insertion, deletion, or conversion. Editing of tRNA anticodons by deamination of adenine to inosine is used both by eukaryotes and prokaryotes to expand the decoding capacity of individual tRNAs. This limits the number of tRNA species required for codon-anticodon recognition. We have identified the Arabidopsis thaliana gene that codes for tRNA adenosine deaminase arginine (TADA), a chloroplast tRNA editing protein specifically required for deamination of chloroplast (cp)-tRNA^Arg(ACG) to cp-tRNA^Arg(ICG). Land plant TADAs have a C-terminal domain similar in sequence and predicted structure to prokaryotic tRNA deaminases and also have very long N-terminal extensions of unknown origin and function. Biochemical and mutant complementation studies showed that the C-terminal domain is sufficient for cognate tRNA deamination both in vitro and in planta. Disruption of TADA has profound effects on chloroplast translation efficiency, leading to reduced yields of chloroplast-encoded proteins and impaired photosynthetic function. By contrast, chloroplast transcripts accumulate to levels significantly above those of wild-type plants. Nevertheless, absence of cp-tRNA^Arg(ICG) is compatible with plant survival, implying that two out of three CGN codon recognition occurs in chloroplasts, though this mechanism is less efficient than wobble pairing.