DNA ligases seal 5-PO 4 and 3-OH polynucleotide ends via three nucleotidyl transfer steps involving ligase-adenylate and DNA-adenylate intermediates. DNA ligases are essential guardians of genomic integrity, and ligase dysfunction underlies human genetic disease syndromes. Crystal structures of DNA ligases bound to nucleotide and nucleic acid substrates have illuminated how ligase reaction chemistry is catalyzed, how ligases recognize damaged DNA ends, and how protein domain movements and active-site remodeling are used to choreograph the end-joining pathway. Although a shared feature of DNA ligases is their envelopment of the nicked duplex as a C-shaped protein clamp, they accomplish this feat by using remarkably different accessory structural modules and domain topologies. As structural, biochemical, and phylogenetic insights coalesce, we can expect advances on several fronts, including (i) pharmacological targeting of ligases for antibacterial and anticancer therapies and (ii) the discovery and design of new strand-sealing enzymes with unique substrate specificities.
DNA LigaseThe discovery of DNA ligases in 1967 by the Gellert, Lehman, Richardson, and Hurwitz laboratories was a watershed event in molecular biology (reviewed in Ref. 1). By joining 3Ј-OH and 5Ј-PO 4 termini to form a phosphodiester, DNA ligases are the sine qua non of genome integrity. They are essential for DNA replication and repair in all organisms. Ligases were critical reagents in the development of molecular cloning and many subsequent ramifications of DNA biotechnology, including molecular diagnostics and SOLiD sequencing methods. Ligases are elegant and versatile enzymes and are enjoying a research renaissance in light of discoveries that most organisms have multiple ligases that either function in DNA replication (by joining Okazaki fragments) or are dedicated to particular DNA repair pathways, such as nucleotide excision repair, base excision repair, single-strand break repair, or the repair of doublestrand breaks via nonhomologous end joining (2-4). Genetic deficiencies in human DNA ligases have been associated with clinical syndromes marked by immunodeficiency, radiation sensitivity, and developmental abnormalities (3). The physiology and division of labor among cellular DNA ligases are the subjects of recent reviews (2-4) and will not be covered here. This minireview will focus on new insights to ligase mechanism and evolution from ligase structure.DNA ligation entails three sequential nucleotidyl transfer steps (Fig. 1). In the first step, nucleophilic attack on the ␣-phosphorus of ATP or NAD ϩ by ligase results in release of PP i or NMN and formation of a covalent ligase-adenylate intermediate in which AMP is linked via a P-N bond to N-of a lysine. In the second step, the AMP is transferred to the 5Ј-end of the 5Ј-phosphate-terminated DNA strand to form DNAadenylate. In this reaction, the 5Ј-phosphate oxygen of the DNA strand attacks the phosphorus of ligase-adenylate, and the active-site lysine is the leaving group. In the thir...