The Transamidosome: A Dynamic Ribonucleoprotein Particle Dedicated to Prokaryotic tRNA-Dependent Asparagine Biosynthesis

Abstract

Asparagine, one of the 22 genetically encoded amino acids, can be synthesized by a tRNA-dependent mechanism. So far, this type of pathway was believed to proceed via two independent steps. A nondiscriminating aspartyl-tRNA synthetase (ND-DRS) first generates a mischarged aspartyl-tRNAAsn that dissociates from the enzyme and binds to a tRNA-dependent amidotransferase (AdT), which then converts the tRNA-bound aspartate into asparagine. We show herein that the ND-DRS, tRNAAsn, and AdT assemble into a specific ribonucleoprotein complex called transamidosome that remains stable during the overall catalytic process. Our results indicate that the tRNAAsn-mediated linkage between the ND-DRS and AdT enables channeling of the mischarged aspartyl-tRNAAsn intermediate between DRS and AdT active sites to prevent challenging of the genetic code integrity. We propose that formation of a ribonucleoprotein is a general feature for tRNA-dependent amino acid biosynthetic pathways that are remnants of earlier stages when amino acid synthesis and tRNA aminoacylation were coupled.

Ancient translation factor is essential for tRNA-dependent cysteine biosynthesis in methanogenic archaea

“…In these processes, the nondiscriminating aaRSs (ND-aaRSs) charge tRNA Asn and tRNA Gln with Asp (24) and Glu (25), respectively; then they are converted to Asn-tRNA Asn (24,26) and Gln-tRNA Gln (27), respectively, by amidotransferases (AdTs). The ND-aaRSs and AdTs can form complexes called transamidosomes (28), which could allow channeling of the mischarged aa-tRNAs from the ND-aaRSs to the AdTs without dissociation from the complexes (29,30). Several different types of transamidosome and regulatory features have been discovered (31).…”

“…(i) The Asn transamidosome in Thermus thermophilus requires tRNA Asn as a scaffold for formation and is stable over the sequential reactions (28,29). It enhances the aminoacylation activity, protects the mischarged Asp-tRNA Asn from deacylation, and increases the overall production of Asn-tRNA Asn (28).…”

Redundant Synthesis of Cysteinyl-tRNACys in Methanosarcina mazei

“…Consistent with other studies in which a common tRNA is recognized by distinct tRNA synthetases, these data were interpreted to support a general hypothesis that aminoacyl-tRNA synthetases evolved in the context of preestablished tRNA identity. Other work on the M. jannaschii pathway has shown that SepRS and SepCysS form a stable binary complex that assists in conversion of the intermediate SeptRNA Cys to Cys-tRNA Cys , a complex reminiscent of the transamidosome particle that facilitates prokaryotic tRNA-dependent asparagine biosynthesis (11,12).…”

Plasmodium Apicoplast Gln-tRNAGln Biosynthesis Utilizes a Unique GatAB Amidotransferase Essential for Erythrocytic Stage Parasites

“…6D). This suggests that the Plasmodium apicoplast may contain a tRNA Gln ⅐ND-GluRS⅐GatA⅐GatB complex, akin to the transamidosomes described in archaea (61) and the eubacterium T. thermophilus (84). Such a complex would prevent challenging the genetic code integrity as demonstrated for tRNA-dependent Asn formation (61).…”