DNA ligase 1 also DNA ligase I, is an enzyme that in humans is encoded by the LIG1 gene. DNA ligase 1 is the only known eukaryotic DNA ligase involved in both DNA replication and repair, making it the most studied of the ligases.

It was known that DNA replication occurred through a double strand break, but the enzyme responsible for ligating the strands back together, and mechanism of action, was unknown until Lehman, Gellert, Richardson, and Hurwitz laboratories, made significant contributions to the discovery of DNA ligase in 1967.

LIG1 encodes a, 120kDa enzyme, 919 residues long, known as DNA ligase 1. The DNA ligase 1 polypeptide contains an N-terminal replication factory-targeting sequence (RFTS), followed by a nuclear localization sequence (NLS), and three functional domains. The three domains consist of an N-terminal DNA binding domain (DBD), and catalytic nucleotidyltransferase (NTase), and C-terminal oligonucleotide / oligosaccharide binding (OB) domains. Although the N-terminus of the peptide has no catalytic activity it is needed for activity within the cells. The N-terminus of the protein contains a replication factory-targeting sequence that is used to recruit it to sites of DNA replication known as replication factories.

Activation and recruitment of DNA ligase 1 seem to be associated with posttranslational modifications. N-terminal domain is completed through phosphorylation of four serine residues on this domain, Ser51, Ser76, and Ser91 by cyclin-dependent kinase (CDK) and Ser66 by casein kinase II (CKII). Phosphorylation of these residues (Ser66 in particular) has been shown to possibly regulate the interaction between the RFTS to the proliferating cell nuclear antigen (PCNA) when ligase 1 is recruited to the replication factories during S-phase. Rossi et al. proposed that when Ser66 is dephosphorylated, the RFTS of ligase 1 interact with PCNA, which was confirmed in vitro by Tom et al. Both data sets provide plausible evidence the N-terminal region of ligase I plays a regulatory role in the enzymes in vivo function in the nucleus. Moreover, the identification of a cyclin binding (Cy) motif in the catalytic C-terminus domain was shown by mutational analysis to play a role in the phosphorylation of serines 91 and 76. Together, the N-terminal serines are substrates of the CDK and CKII, which appear to play an important regulatory role DNA ligase I recruitment to the replication factory during S-phase of the cell cycle.

LIG1 encodes DNA ligase 1, which functions in DNA replication and the base excision repair process.

Eukaryotic DNA ligase 1 catalyzes a reaction that is chemically universal to all ligases. DNA ligase 1 utilizes adenosine triphosphate (ATP) to catalyze the energetically favorable ligation events in both DNA replication and repair. During the synthesis phase (S-phase) of the eukaryotic cell cycle, DNA replication occurs. DNA ligase 1 is responsible for joining Okazaki fragments formed during discontinuous DNA synthesis on the DNA's lagging strand after DNA polymerase δ has replaced the RNA primer nucleotides with DNA nucleotides. If the Okazaki fragments are not properly ligated together, the unligated DNA (containing a ‘nick’) could easily degrade to a double strand break, a phenomenon known to cause genetic mutations. In order to ligate these fragments together, the ligase progresses through three steps:

During adenylylation, there is a nucleophilic attack on the alpha phosphate of ATP from a catalytic lysine resulting in the production of inorganic pyrophosphate (PPi) and a covalently bound lysine-AMP intermediate in the active site of DNA ligase 1.

During the AMP transfer step, the DNA ligase becomes associated with the DNA, locates a nick and catalyzes a reaction at the 5’ phosphate site of the DNA nick. An anionic oxygen on the 5’ phosphate of the DNA nick serves as the nucleophile, attacking the alpha phosphate of the covalently bound AMP causing the AMP to be covalently bound intermediate (DNA-AMP intermediate).