DNA ligase is an enzyme that joins adjacent ends of DNA by forming a phosphodiester bond in its sugar–phosphate backbone. Its characteristic reaction connects a 3′-hydroxyl group to a 5′-phosphate group, restoring continuity to a broken strand. DNA ligases participate in DNA replication, DNA repair, and recombination, and purified ligases are important tools for assembling DNA in laboratories. They seal existing DNA ends rather than synthesize a new strand from individual nucleotides. (pmc.ncbi.nlm.nih.gov)
Substrates and reaction mechanism
A typical cellular substrate is a nick: an interruption in one strand of double-stranded DNA without missing nucleotides at the junction. The complementary strand maintains the alignment of the two ends. A nick therefore differs from a gap, which requires replacement of missing DNA before ordinary nick sealing can occur. Some ligases also join separate double-stranded DNA fragments when their ends are appropriately positioned. (pmc.ncbi.nlm.nih.gov)
Ligation follows three chemical steps:
- Enzyme adenylation. An AMP group becomes covalently attached to a conserved lysine residue in the ligase.
- DNA adenylation. AMP is transferred from the enzyme to the DNA’s 5′-phosphate, producing an activated DNA–adenylate intermediate.
- Strand sealing. The neighboring 3′-hydroxyl attacks the activated phosphate, forming the phosphodiester bond and releasing AMP.
This sequence couples activation of the DNA end to formation of the final covalent bond. It requires a nucleotide cofactor and a divalent metal ion, commonly magnesium, which assists the catalytic chemistry. (pmc.ncbi.nlm.nih.gov)
The reaction is distinct from that of DNA polymerase. Polymerases extend a strand by incorporating nucleotides, whereas ligases connect ends that already exist. Thus, DNA synthesis and ligation perform complementary tasks: synthesis supplies the sequence, and ligation completes backbone continuity. (nature.com)
Cofactor classes and distribution
DNA ligases are commonly divided into ATP-dependent and NAD-dependent classes. ATP-dependent enzymes obtain their adenylate group from adenosine triphosphate, releasing pyrophosphate during enzyme activation. NAD-dependent enzymes use oxidized nicotinamide adenine dinucleotide and release nicotinamide mononucleotide. Both classes subsequently use DNA adenylation and strand sealing to complete the reaction. (pmc.ncbi.nlm.nih.gov)
Mammalian ligases are ATP-dependent. NAD-dependent ligases include the principal replicative enzymes of many bacteria, but the distinction is not an absolute division between bacterial and eukaryotic biology: some bacteria encode ATP-dependent ligases in addition to their NAD-dependent enzyme. Laboratory reagents likewise include both classes, with different substrate preferences and operating conditions. (nature.com)
Molecular structure
DNA ligases are proteins whose catalytic regions bind the nucleotide cofactor and organize the DNA ends for reaction. Their conserved catalytic machinery is supplemented by domains that recognize DNA or interact with other proteins. Structural experiments show that ligation involves substantial domain movements rather than a fixed active-site configuration. (pmc.ncbi.nlm.nih.gov)
X-ray crystallography revealed that human ligase I can completely encircle nicked DNA. Its DNA-binding, adenylation, and oligonucleotide-binding-fold domains form a ring around the substrate, distorting the duplex to expose and position the broken ends. Structures of human ligase IV show open and closed intermediate states, illustrating how changes in domain arrangement accompany different stages of catalysis. (nature.com)
Functions in mammalian cells
Mammalian cells contain three principal ATP-dependent DNA ligases, conventionally designated I, III, and IV. Their roles depend on localization, interacting proteins, and the DNA-processing pathway in which they operate. (nature.com)
DNA ligase I, encoded by the gene LIG1, joins Okazaki fragments during lagging-strand replication. It also participates in recombination and base excision repair. In replication, ligation completes the joining of fragments after their ends have been processed into a sealable junction. (nature.com)
DNA ligase III has nuclear and mitochondrial forms. Its nuclear form interacts with the repair scaffold XRCC1, while its mitochondrial activity supports maintenance of DNA within the mitochondrion. Genetic experiments in mammalian cells demonstrated that ligase III is critical for mitochondrial DNA maintenance but is not universally indispensable for XRCC1-dependent nuclear repair; other ligase activity can support that repair in the tested systems. (nature.com)
DNA ligase IV works with XRCC4 in classical non-homologous end joining, which repairs double-strand breaks. It is also essential for V(D)J recombination, the DNA rearrangement process that assembles antigen-receptor genes. Its role is the joining stage within a larger complex that recognizes and processes broken DNA ends. (ncbi.nlm.nih.gov)
Laboratory applications
DNA ligases enable construction of recombinant DNA, including insertion of DNA fragments into plasmid vectors. T4 DNA ligase, associated with bacteriophage T4, is widely used because it seals nicks and joins both cohesive and blunt double-stranded ends. Cohesive ends have complementary single-stranded overhangs; blunt ends lack such overhangs. Joining requires suitable terminal chemistry, including a 5′-phosphate at each junction to be sealed. (neb.com)
Ligases are not interchangeable. For example, thermostable Taq DNA ligase uses NAD and joins adjacent DNA strands aligned on a complementary template. Its discrimination against incorrectly paired junctions makes it useful in ligation-based detection of sequence variants. Ligation efficiency and specificity depend on the enzyme, end structure, buffer, and reaction temperature; an enzyme suited to template-directed nick sealing need not efficiently join free DNA fragments. (intl.neb.com)