Where have all the inosines gone? Conflicting evidence for A-to-I editing of the anticodon of higher eukaryotic tRNAACGArg questions the dogma of a universal wobble-mediated decoding of CGN codons.

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 tRNAACGArg 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 tRNAACGArg. 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. © 2016 IUBMB Life, 68(6):419-422, 2016.

The absence of A-to-I editing in the anticodon of plant cytoplasmic tRNA (Arg) ACG demands a relaxation of the wobble decoding rules.

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.