The transamidosome: A dynamic ribonucleoprotein particle dedicated to prokaryotic tRNA-Dependent asparagine biosynthesis

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-tRNA(Asn) 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, tRNA (Asn), and AdT assemble into a specific ribonucleoprotein complex called transamidosome that remains stable during the overall catalytic process. Our results indicate that the tRNA(Asn) -mediated linkage between the ND-DRS and AdT enables channeling of the mischarged aspartyl-tRNA(Asn) 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.