4974 Nucleic Acids Research, 2012, Vol. 40, No. 11 Transamidosomes are editing-like systems that enlarged the genetic code AaRSs are responsible for matching a tRNA and its cognate amino acid. Many of these enzymes acquired catalytic peculiarities that nowadays ensure homologous aminoacyl-tRNA production, with an error rate compat- ible with survival of the cell (4,47,48). In spite of their speciﬁcity, aaRSs are known to use non-cognate amino acid or metabolic precursors to form mischarged aminoacyl-tRNAs. ValRS and IleRS can respectively produce Thr-tRNAVal and Val-tRNAIle (49,50), and LeuRS can misacylate isoleucine, methionine, homocyst- eine, a-amino butyrate and norvaline onto tRNALeu (51). The release of mischarged aminoacyl-tRNA is normally prevented by correction mechanisms [either hydrolysis of the non-cognate aminoacyl AMP intermediate (pre-transfer editing) or hydrolysis of the ester linkage of the mischarged aminoacyl-tRNA (post-transfer editing)] (4,49–53). Post-transfer correction is thought to rely on aminoacyl-tRNA dissociation as the rate-limiting step providing time for the misacylated aminoacyl-tRNA acceptor arm to shift from the active site to the editing site. Interestingly, this mechanism completely relies on the presence of the cognate tRNA (54). Post-transfer editing can also be achieved by autonomous trans-editing factors (4). Indeed, as an example, ProRS and tRNAPro form a stable complex together with YbaK to avoid accumulation of Cys-tRNAPro and subsequent mistranslation (55). Asn and Gln-transamidosomes constitute an assembly of an ND-aaRS (ND-AspRS or ND-GluRS/GluRS2), a ‘non-cognate’ tRNA (tRNAAsn or tRNAGln) and a homeotopic modiﬁcation enzyme (56); together these components ensure accurate synthesis of the cognate aminoacyl-tRNA pair. Since they reduce toxicity of misacylated species by preventing their release into the cellular medium, Asn- and Gln-transamidosomes could also be considered as kinetic systems related to editing systems that increase the faithfulness of aminoacylation and protect the integrity of the genetic code (Supplementary Figure S2 and 21,24). In H. pylori, the Asn-transamidosome mimics a post-transfer editing system, in which GatCAB would play a role similar to a trans-editing factor, except that the misacylated aminoacyl-tRNA does not get hydrolyzed but is rather modiﬁed into a cognate aminoacyl-tRNA. In line with this scenario, it is noteworthy that indirect pathways are thought to be ancient routes that allowed ancient ‘Asp’ and ‘Glu’ codons to be shifted into ‘Asn’ and ‘Gln’ codons and could thus be regarded as codon reassignment systems (24). Indeed, before GatCAB appeared, tRNAAsn and tRNAGln were considered as normal ND-AspRS and GluRS substrates. However, among past Asp-tRNAs, the present-day tRNAAsn emerged with a different kinetic behavior that favored its modiﬁcation by GatCAB. While editing systems restricted and still restrict the com- binations of small molecules that can be linked to tRNAs, Asn and Gln-transamidosomes were ribonucleoprotein systems that enlarged the genetic code by shifting some Asp-tRNA and Glu-tRNA species into Asn-tRNAAsn and Gln-tRNAGln. GatCAB is a node for transamidation Helicobacter pylori belongs to a subclass of organisms that lack both GlnRS and AsnRS, in which GatCAB plays a dual role in glutaminylation and asparaginylation. In contrast, archaea possess two tRNA-dependent amidotransferases each speciﬁc for one pathway (57). In H. pylori, GatCAB is able to bind very efﬁciently to uncharged tRNAAsn (this study) but also to tRNAGln (24) with equivalent afﬁnities (2.1 and 2.0 mM, respectively) (Supplementary Scheme S1). This capacity provides two types of tRNA-presentation complexes: tRNPC and the tRNAGln-presentation complex (tRQPC), which may rep- resent a way to target GatCAB toward ND-AspRS and asparaginylation on one hand and GluRS2 (the enzyme speciﬁc for tRNAGln misacylation in Hp) (25,34) and glutaminylation on the other hand (Supplementary Scheme S1). This need to share GatCAB between two distinct aaRSs could explain the reason why the Asn- and Gln-transamidosomes are both more dynamic in H. pylori than in T. thermophilus (22) or T. maritima (23). Hence, transamidosomes may be adapted to the metabolic context and physiology of the organism from which they originate. SUPPLEMENTARY DATA Supplementary Data are available at NAR Online: Sup- plementary Table 1, Supplementary Scheme 1 and Supplementary Figures 1–3. ACKNOWLEDGEMENTS We thank Prof. E. Westhof and Dr R. Giege´ (IBMC) for support, Prof. R. Ch eˆnevert (Universite´ Laval, Que´ bec) for providing Asp-ol-AMP and M. Braye´ (IBMC) for technical assistance. FUNDING Natural Sciences and Engineering Research Council of Canada (9597-2010 to J.L.); Fond de Recherche sur la Nature et les Technologies du Que´ bec (PR-105093 to J.L., Robert Ch eˆnevert and Paul H. Roy, and graduate fellowship to J.L.H.); Regroupement Que´ becois de Recherche sur la Fonction, la Structure et l’Inge´ nierie des Proteines (PROTEO) (graduate fellowship to J.L.H.); ‘Ministe` re du De´ veloppement E´ conomique, de l’Innovation et de l’Exportation (MDEIE) du Que´ bec’ [grant PSR-SIIRI-095/ESFARN to J.L., Sheng-Xiang Lin and Robert Cheˆ nevert, in collaboration with Prof. You-Xin Jin at the Shanghai Institute of Biological Sciences (SIBS), Chinese Academy of Sciences]; Ministe` re de l’Education Nationale, de la Recherche et de la Technologie (graduate fellowship to F.F.); Universite´ de Strasbourg, Centre National de la Recherche Scientiﬁque and Association pour la Recherche sur le Cancer (to D.K.) and National Institutes of Health (R01GM071480 to T.L.H.). Funding for open access charge: Centre National de la Recherche