Structural basis of tRNA agmatinylation essential for AUA codon decoding

TiaS catalyzes the transfer of agmatine onto the first position cytidine of the tRNAIle2 anticodon in archaea, ensuring proper translation of the matching codon. Now the crystal structures of the TiaS–tRNAIle2 complex with ATP, or with AMPCPP and agmatine, reveal a novel kinase domain and show how TiaS selects the correct tRNA while segregating the target cytidine until agmatine is bound. The cytidine at the first position of the anticodon (C34) in the AUA codon-specific archaeal tRNA Ile2 is modified to 2-agmatinylcytidine (agm 2 C or agmatidine), an agmatine-conjugated cytidine derivative, which is crucial for the precise decoding of the genetic code. Agm 2 C is synthesized by tRNA Ile -agm 2 C synthetase (TiaS) in an ATP-dependent manner. Here we present the crystal structures of the Archaeoglobus fulgidus TiaS–tRNA Ile2 complexed with ATP, or with AMPCPP and agmatine, revealing a previously unknown kinase module required for activating C34 by phosphorylation, and showing the molecular mechanism by which TiaS discriminates between tRNA Ile2 and tRNA Met . In the TiaS–tRNA Ile2 –ATP complex, C34 is trapped within a pocket far away from the ATP-binding site. In the agmatine-containing crystals, C34 is located near the AMPCPP γ-phosphate in the kinase module, demonstrating that agmatine is essential for placing C34 in the active site. These observations also provide the structural dynamics for agm 2 C formation.