TRANSFER-RNA DEPENDENT AMIDOTRANSFERASES KEY ENZYMES FOR ASN-TRNA AND GLU-TRNA SYNTHESIS IN NATURE

Abstract

Transfer RNA-dependent amidotransferases (AdTs) are essential for Gln-tRNA Gln and Asn-tRNA Asn synthesis in organisms lacking glutaminyl-or asparaginyl-tRNA synthetase. In the presence of ATP and an amide donor, AdT catalyzes the formation of Asn-tRNA Asn or Gln-tRNA Gln by amidating the misaminoacylated tRNA species Asp-tRNA Asn or Glu-tRNA Gln , which are synthesized by a non-discriminating aspartyl-or glutamyl-tRNA synthetase. AdTs are widespread and have been found in all domains of life: archaea, bacteria, and eukaryotic organelles. Two types of AdTs have been identified so far, a heterotrimeric enzyme (encoded by gatCAB) found in both bacteria and archaea and some organelles, and a heterodimeric one (encoded by gatDE) found exclusively in archaea. While GatCAB amidates both Asp-tRNA Asn and Glu-tRNA Gln in vitro, GatDE recognizes only Glu-tRNA Gln . GatA shares sequence similarity with amidases and GatD resembles asparaginases. Therefore, it appears that AdTs have recruited enzymes of amino acid catabolism to carry out the glutaminase reaction, which provides the active ammonia for amidation. tRNA-dependent Asn-tRNA formation by GatCAB can also be used as an organism's sole biosynthetic route to asparagine. The intricate relationship displayed between AdTs and amino acid metabolism enzymes suggests that they may share evolutionary origins. This chapter discusses the significance of the universal distribution of the transamidation route to amide aminoacyl-tRNA biosynthesis and the multifaceted roles displayed by the extraordinary enzymes that carry out these complex reactions.