A single tRNA base pair mediates bacterial tRNA-dependent biosynthesis of asparagine

In many prokaryotes and in organelles asparagine and glutamine are formed by a tRNA-dependent amidotransferase (AdT) that catalyzes amidation of aspartate and glutamate, respectively, mischarged on tRNAAsn and tRNAGln. These pathways supply the deficiency of the organism in asparaginyl- and glutaminyl-tRNA synthtetases and provide the translational machinery with Asn-tRNAAsn and Gln-tRNAGln. So far, nothing is known about the structural elements that confer to tRNA the role of a specific cofactor in the formation of the cognate amino acid. We show herein, using aspartylated tRNAAsn and tRNAAsp variants, that amidation of Asp acylating tRNAAsn is promoted by the base pair U1–A72 whereas the G1–C72 pair and presence of the supernumerary nucleotide U20A in the D-loop of tRNAAsp prevent amidation. We predict, based on comparison of tRNAGln and tRNAGlu sequence alignments from bacteria using the AdT-dependent pathway to form Gln-tRNAGln, that the same combination of nucleotides also rules specific tRNA-dependent formation of Gln. In contrast, we show that the tRNA-dependent conversion of Asp into Asn by archaeal AdT is mainly mediated by nucleotides G46 and U47 of the variable region. In the light of these results we propose that bacterial and archaeal AdTs use kingdom-specific signals to catalyze the tRNA-dependent formations of Asn and Gln.