The ribosome allows thebest possible spatial arrangement of the various partners and ensures catalysisbut the adaptor molecule which acts as a link between codesof mRNA and amino acids of polypeptide is the tRNA. For play this major role, tRNAs have two distinctcharacteristics corresponding to two different genetic codes, the anticodon andthe operational codes. The latter which is mainly embodied in the acceptor-stemallows to bind covalently and with a high specificity an amino acid to a tRNA,a reaction catalyzed by a specific aminoacyl-tRNA synthetase (aaRS). Theoperational code might have actually predated the “classic” codeassociated with anticodons (Schimmelet al., 1993) . Moreover, thetRNAs exhibit diversity in uniqueness, all of themmust be similar for entering the ribosome machinery; so, they generally lookstructurally homogeneous, especially in their secondary and tertiary structureseven if “non-classical” tRNAs are known. Moreover, cloverleafstructure and especially the tertiary interaction network governing theL-shaped tRNA architecture imply conserved and semi-conserved bps and nts.

Onthe other side, each type of tRNA structures must interact specifically with aaRSsand post-transcriptional modification enzymes which implies that parts of theirsequences and of their structures (as the V-R size) allow to distinguish them.Reduced bacterial and most organellegenomes do not encode the full set of 32 tRNA species required to read alltriplets of the standard genetic code according to the conventional wobblerules. Superwobbling, in which a single tRNA species that contains modificationsof the anticodon loop, such as an hypermodified uridine in the wobble position 34of the anticodon, allows to read all 4 nts in the third codon position, hasbeen suggested as a possible mechanism for how reduced tRNA sets may be functional(Agris et al., 2007), e.g.

, in several metazoan mtDNAs only a total of 22 tRNAs aresufficient to recognize all codons (two tRNAs each for serine andleucine and one tRNA for each of the other 18 amino acids). However, superwobbling induces areduced translational efficiency, which could explain why most organisms haveadopted pairs of isoaccepting tRNAs over the superwobbling mechanism (Rogalski et al., 2008) .

As,e.g., in Cnidaria (sea anemones, corals, etc…) orChaetognatha (marine invertebrates), current mtDNAs have lost several of their trn genes and the absence of anapparently full set of mt-trn geneshas also been mentioned (Huot et al., 2014) .

Studies have investigated the fate of missingtRNAs and their corresponding aaRSs (Haen et al. 2010) and inmany cases, the lost tRNAs are functionally replaced by imported of nucleus-encoded tRNAs (Huotet al., 2014) . However, a recentstrategy strongly suggests that efficient reanalyzes allow to found severaltRNA-like structures (TLS) which can be efficient tRNAs (Barthélémy and Seligmann, 2016).


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