ATP-dependent RNA ligases are agents of RNA repair that join 3′-OH and 5′-PO 4 RNA ends. Naegleria gruberi RNA ligase (NgrRnl) exemplifies a family of RNA nick-sealing enzymes found in bacteria, viruses, and eukarya. Crystal structures of NgrRnl at three discrete steps along the reaction pathway-covalent ligase-(lysyl- 2+ complexhighlight a two-metal mechanism of nucleotidyl transfer, whereby (i) an enzyme-bound "catalytic" metal coordination complex lowers the pK a of the lysine nucleophile and stabilizes the transition state of the ATP α phosphate; and (ii) a second metal coordination complex bridges the β-and γ-phosphates. The NgrRnl N domain is a distinctively embellished oligonucleotide-binding (OB) fold that engages the γ-phosphate and associated metal complex and orients the pyrophosphate leaving group for in-line catalysis with stereochemical inversion at the AMP phosphate. The unique domain architecture of NgrRnl fortifies the theme that RNA ligases have evolved many times, and independently, by fusions of a shared nucleotidyltransferase domain to structurally diverse flanking modules. The mechanistic insights to lysine adenylylation gained from the NgrRnl structures are likely to apply broadly to the covalent nucleotidyltransferase superfamily of RNA ligases, DNA ligases, and RNA capping enzymes.NζRNA repair | covalent nucleotidyltransferase | lysyl-AMP B iochemically diverse RNA repair systems rely on RNA ligases to maintain or manipulate RNA structure in response to purposeful RNA breakage events (1-5). RNA breaks destined for repair are inflicted by sequence-specific or structure-specific endoribonucleases during physiological RNA processing (e.g., tRNA splicing; kinetoplast mRNA editing) and under conditions of cellular stress (e.g., virus infection, starvation, unfolded protein response). RNA cleavage can occur either: (i) by a transesterification mechanism (generally metal-independent) that yields 2′,3′-cyclic-PO 4 and 5′-OH ends; or (ii) via a hydrolytic mechanism (typically metaldependent) that leaves 3′-OH and 5′-PO 4 ends. RNA repair enzymes capable of sealing 2′,3′-cyclic-PO 4 /5′-OH breaks or 3′-OH/5′-PO 4 breaks are distributed widely in all phylogenetic domains of life.ATP-dependent RNA ligases join 3′-OH and 5′-PO 4 RNA termini via a series of three nucleotidyl transfer steps similar to those of DNA ligases (6). In step 1, RNA ligase reacts with ATP to form a covalent ligase-(lysyl-Nζ)-AMP intermediate plus pyrophosphate. In step 2, AMP is transferred from ligase-adenylate to the 5′-PO 4 RNA end to form an RNA-adenylate intermediate, AppRNA. In step 3, ligase catalyzes attack by an RNA 3′-OH on the RNA-adenylate to seal the two ends via a phosphodiester bond and release AMP.The autoadenylylation reaction of RNA ligases is performed by a nucleotidyltransferase (NTase) domain that is conserved in ATP-dependent and NAD + -dependent DNA ligases and GTPdependent mRNA capping enzymes (6, 7). The NTase domain includes six peptide motifs (I, Ia, III, IIIa, IV, and V) that form the nucleotide-bin...