DNAligas

DNAligas refers to a family of enzymes that catalyze the sealing of nicks in double-stranded DNA by forming a phosphodiester bond between adjacent nucleotides. The reaction is essential for DNA replication and various DNA repair pathways, and without ligases, DNA strands remain broken.

Most DNAligas operate via a two-step mechanism that proceeds through an enzyme-adenylate intermediate. The ligase is activated by transfer of an adenylyl group from either ATP or NAD+ to a conserved lysine in the active site. The adenylyl group is then transferred to the 5' phosphate at the nick, creating a 5'-adenylylated end. The adjacent 3' hydroxyl attacks the activated phosphate, forming the phosphodiester bond and releasing AMP or adenosine monophosphate.

Type-specific roles: In eukaryotes the principal replicative enzyme is DNA Ligase I, which seals nicks on the lagging strand; DNA Ligase III operates in base excision repair and mitochondrial DNA maintenance, often in complex with XRCC1; DNA Ligase IV participates in non-homologous end joining by partnering with XRCC4. In bacteria, NAD+-dependent ligases (LigA) perform similar functions and some species also have LigB. Archaea and some eukaryotes use ATP-dependent ligases. The function of DNAligas is complemented by other repair factors; defects can reduce genome stability and sensitivity to DNA damage.

Biotechnological use: DNA ligases are widely used to join DNA fragments in cloning, vector construction, and forensic applications. In laboratories, T4 DNA ligase from bacteriophage T4 is a commonly used enzyme for ligating DNA fragments at moderate temperatures, whereas thermostable ligases enable ligation during PCR-based protocols.

Clinical and evolutionary notes: Mutations in human ligases, particularly LIG4, LIG1, or LIG3, can compromise DNA repair and are linked to developmental disorders, immune defects, and cancer susceptibility. DNAligas are highly conserved across life and reflect the evolution of DNA repair pathways that sustain genome integrity.