Significance This season marks the 50th anniversary of the discovery of polynucleotide ligases, the sine qua non enzymes of nucleic acid repair and the enabling reagents of molecular biology, reported in a series of seminal papers in PNAS. Ligases react with ATP or NAD + to form a covalent enzyme–adenylate intermediate in which AMP is linked via a P–N bond to a lysine. This work reports crystal structures of the Michaelis complexes of an exemplary ATP-dependent RNA ligase (bacteriophage T4 Rnl1) and an NAD + -dependent DNA ligase ( Escherichia coli LigA) that illuminate the chemical and structural basis for lysine adenylylation, via distinctive two-metal (ATP) and one-metal (NAD + ) mechanisms.