This region is found in many but not all ATP-dependent DNA ligase enzymes (EC:6.5.1.1). It is thought to be involved in DNA binding and in catalysis. In human DNA ligase I (Swiss:P18858), and in Saccharomyces cerevisiae (Swiss:P04819), this region was necessary for catalysis, and separated from the amino terminus by targeting elements. In vaccinia virus (Swiss:P16272) this region was not essential for catalysis, but deletion decreases the affinity for nicked DNA and decreased the rate of strand joining at a step subsequent to enzyme-adenylate formation [1].

1: Domain structure of vaccinia DNA ligase. Sekiguchi J, Shuman S; Nucleic Acids Res 1997;25:727-734. PMID:9016621