The absence of A-to-I editing in the anticodon of plant cytoplasmic tRNA^Arg_ACGdemands a relaxation of the wobble decoding rules

Abstract

It is a prevalent concept that, in line with the Wobble Hypothesis, those tRNAs having an adenosine in the first position of the anticodon become modified to an inosine at this position. Sequencing the cDNA derived from the gene coding for cytoplasmic tRNA (Arg) ACG from several higher plants as well as mass spectrometric analysis of the isoacceptor has revealed that for this kingdom an unmodified A in the wobble position of the anticodon is the rule rather than the exception. In vitro translation shows that in the plant system the absence of inosine in the wobble position of tRNA (Arg) does not prevent decoding. This isoacceptor belongs to the class of tRNA that is imported from the cytoplasm into the mitochondria of higher plants. Previous studies on the mitochondrial tRNA pool have demonstrated the existence of tRNA (Arg) ICG in this organelle. In moss the mitochondrial encoded distinct tRNA (Arg) ACG isoacceptor possesses the I34 modification. The implication is that for mitochondrial protein biosynthesis A-to-I editing is necessary and occurs by a mitochondrion-specific deaminase after import of the unmodified nuclear encoded tRNA (Arg) ACG.

Life without tRNAArg–adenosine deaminase TadA: evolutionary consequences of decoding the four CGN codons as arginine in Mycoplasmas and other Mollicutes

“…These I 34 R 35 -containing tRNAs are found in the decoding family boxes using three or four synonymous codons (Leu, Ile, Val, Ser, Pro, Thr and Ala) (11,15). However, in Arabidopsis thaliana and other land plants, the same cytoplasmic Tad2/Tad3 deaminase does not deaminate the wobble A 34 of cytoplasmic tRNA Arg A CG , but only those of the other A 34 R 35 -containing tRNAs (16). This raises the question of how the Arg-CG N codons in plant cytoplasmic mRNAs are translated into arginine.…”

Identity elements for the aminoacylation of metazoan mitochondrial tRNAArghave been widely conserved throughout evolution and ensure the fidelity of the AGR codon reassignment

“…We have examined C. elegans mitochondrial tRNA Arg ACG by reverse transcription followed by PCR and sequencing. As in the case of cytoplasmic tRNA Arg ACG in plants 38 and in insects (unpublished results), the first position of the anticodon is not deaminated and that, therefore, translation of all arginine codons by this isoacceptor must rely on non-standard wobble interactions. The presence of A34 in the cognate tRNA did not influence the aminoacylation activity by the insect arginyl-tRNA synthetase so that the loss of arginine acceptance by the C. elegans tRNA Arg ACG may be indicative of a modulated recognition mechanism for these isoacceptors.…”

“…61 Reverse transcription analysis of tRNA was performed as described. 38 Total C. elegans RNA was kindly provided by Dr. E. Schulze (Freiburg).…”

“…1), A34 could fully replace U34 in the recognition process in the context of the insect tRNA structure. Since, despite evidence to the contrary, 38 it is considered that in tRNAs A34 is generally deaminated to inosine, 39 a potential involvement of inosine in tRNA recognition required further investigation. Reverse transcription of total C. elegans RNA with a primer specific for tRNAs, followed by PCR with a tRNA Arg ACG -specific primer and sequencing, gave no indication for the presence of an inosine- derived G34 40 (Fig.…”

Two-subunit enzymes involved in eukaryotic post-transcriptional tRNA modification

“…All 6 (of 7) S. cerevisiae tRNA species with an encoded A 34 residue that have been examined have I 34 , and all eukaryotic cytoplasmic tRNAs with an encoded A 34 that have been examined have the I 34 modification 13 , with the exception of wheat tRNA Arg(ACG) . 41 I 34 increases the reading capacity of tRNAs, allowing decoding of codons ending in U and C, and often A, 42,43 and lack of the modification gene is lethal in S. cerevisiae, S. pombe, and E. coli. [44][45][46] In S. cerevisiae, I 34 is formed by the Sc Tad2-Tad3 protein pair.…”