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.

Identity elements determine the accurate recognition between tRNAs and aminoacyl-tRNA synthetases. The arginine system from yeast and Escherichia coli has been studied extensively in the past. However, information about the enzymes from higher eukaryotes is limited and plant aminoacyltRNA synthetases have been largely ignored in this respect. We have designed in vitro tRNA transcripts, based on the soybean tRNA Arg primary structure, aiming to investigate its specific aminoacylation by two recombinant plant arginyl-tRNA synthetases and to compare this with the enzyme from E. coli. Identity elements at positions 20 and 35 in plants parallel those previously established for bacteria. Cryptic identity elements in the plant system that are not revealed within a tRNA Arg consensus sequence compiled from isodecoders corresponding to nine distinct cytoplasmic, mitochondrial and plastid isoaccepting sequences are located in the acceptor stem. Additionally, it has been shown that U20a and A38 are essential for a fully efficient cognate E. coli arginylation, whereas, for the plant arginyl-tRNA synthetases, these bases can be replaced by G20a and C38 with full retention of activity. G10, a constituent of the 10 : 25 : 45 tertiary interaction, is essential for both plant and E. coli activity. Amino acid recognition in terms of discriminating between arginine and canavanine by the arginyl-tRNA synthetase from both kingdoms may be manipulated by changes at different sites within the tRNA structure.

Codon-anticodon recognition between triplets of an mRNA and a specific tRNA is the key element in the translation of the genetic code. In general, the precision of this process is dominated by a strict Watson-Crick base-pairing scheme. However, the degeneracy of the genetic code led Crick to propose the Wobble Hypothesis, permitting a less restraining interaction with the third base of the codon and involving the participation of inosine for decoding C-ending codons. The concept that the anticodon base A34 of tRNA Arg ACG in all eukaryotes, eubacteria, and plant chloroplasts is converted to I34 is firmly anchored in the literature despite conflicting evidence for its existence in higher eukaryote cytoplasmic tRNA Arg ACG . Here, we provide additional data and summarize the arguments favoring and contradicting post-transcriptional deamination of this position. A hypothesis that resolves the apparent conflict is proposed.