Mammalian DNA ligase II

DNA ligases are essential for DNA replication, repair and recombination. Two DNA ligases have been described previously in mammalian cells and other eukaryotes. DNA ligase I has been purified and the cDNA encoding the enzyme has been cloned. In this study, DNA ligase II has been purified from bovine thymus and it has been confirmed that it is distinct from DNA ligase I in its chromatographic, biochemical and catalytic properties. Internal Lys C peptides have been sequenced from apparently homogeneous DNA ligase II. These show no homology to the predicted amino acid sequence of DNA ligase I. However, both analysis of the peptide sequences and two-dimensional gel electrophoresis showed there to be two proteins present in this DNA ligase II preparation. One of these proteins has been identified as cytovillin, an abundant structural protein. Purification from bovine liver gave a source of DNA ligase II that was not contaminated with cytovillin. A radiolabelled reaction intermediate can be generated for the DNA ligases and in this way it has been possible to compare the active site peptides by electrophoretic techniques. This has confirmed that DNA ligase II is distinct from DNA ligase I. A third DNA ligase, DNA ligase III, was identified in the laboratory during the course of this work, and it has been possible to establish a range of substrate specificities for this enzyme that is different from that of either DNA ligase I or II. Comparison of the active site peptides of DNA ligases II and III has indicated a degree of similarity between these two enzymes and suggests they may be related, although both are unrelated to DNA ligase I.