DNA ligase
Since the LIG3 gene encodes the only DNA ligase in mitochondria, inactivation of the LIG3 gene results in loss of mitochondrial DNA that in turn leads to loss of mitochondrial function. Fibroblasts with inactivated Lig3 gene can be propagated in the media supplemented with uridine and pyruvate. However, these cells lack mtDNA. Physiological levels of mitochondrial DNA ligase III appear excessive, and cells with 100-fold reduced mitochondrial content of mitochondrial DNA ligase III-alpha maintain normal mtDNA copy number. The essential role of DNA ligase III-alpha in mitochondrial DNA metabolism can be fulfilled by other DNA ligases, including the NAD-dependent DNA ligase of "E. coli", if they are targeted to mitochondria. Thus, viable cells that lack nuclear DNA ligase III-alpha can be generated. While DNA ligase I is the predominant enzyme that joins Okazaki fragments during DNA replication, it is now evident that the DNA ligase III-alpha/XRCC1 complex enables cells that either lack or have reduced DNA ligase I activity to complete DNA replication. Given the biochemical and cell biology studies linking the DNA ligase III-alpha/XRCC1 complex with excision repair and the repair of DNA single strand breaks, it was surprising that the cells lacking nuclear DNA ligase III-alpha did not exhibit significantly increased sensitivity to DNA damaging agent. These studies suggest that there is significant functional redundancy between DNA ligase I and DNA ligase III-alpha in these nuclear DNA repair pathways. In mammalian cells, most DNA double strand breaks are repaired by DNA ligase IV-dependent non-homologous end joining (NHEJ). DNA ligase III-alpha participates in a minor alternative NHEJ pathway that generates chromosomal translocations. Unlike the other nuclear DNA repair functions, it appears that the role of DNA ligase III-alpha in alternative NHEJ is independent of XRCC1.