Catalyzes the formation of a phosphodiester bond between the 5'-phosphoryl group and the 3'-hydroxyl group of two double-stranded DNA fragments. ATP is required for this reaction. Ligation of blunt-ended DNA is greatly stimulated by polyethylene glycol. Applications include self-circularization of linear DNA prior to transformation, ligation of insert to vector, and ligation of synthetic linkers to blunt-ended DNA. Each predispensed, single-reaction, ambient-temperature-stable tube contains a glassified reaction mix which, when reconstituted to a final reaction volume of 20 μl, yields a solution of reaction buffer, T4 DNA ligase, and ATP.