DNA ligases join single-strand breaks in double-stranded DNA by catalyzing the formation of a phosphodiester bond between adjacent 5'-phosphate and 3'-hydroxyl termini. Their function is essential for maintaining genome integrity in the replication, recombination and repair of DNA. High flexibility is important for the function of DNA ligase molecules. Two types of overall conformations of archaeal DNA ligase that depend on the relative position of the OB-fold domain have previously been revealed: closed and open extended conformations. The structure of ATP-dependent DNA ligase from Thermococcus sp. 1519 (LigTh1519) in the crystalline state determined at a resolution of 3.02 Å shows a new relative arrangement of the OB-fold domain which is intermediate between the positions of this domain in the closed and the open extended conformations of previously determined archaeal DNA ligases. However, small-angle X-ray scattering (SAXS) measurements indicate that in solution the LigTh1519 molecule adopts either an open extended conformation or both an intermediate and an open extended conformation with the open extended conformation being dominant.