A specific endonuclease which co-purifies with calf thymus terminal deoxynucleotidyl transferase

An endonuclease which introduces single nicks in superhelical DNA co-purifies with the enzyme, terminal deoxynucleotidyl transferase. This activity is found in all homogeneous preparations of terminal transferase tested, and remains associated with the polymerase activity during additional fractionation methods. Kinetic data suggest that the nuclease and polymerase occupy distinct active sites, although multifunctionality has not been proven. However, the ability of the polymerase to synthesize oligodeoxynucleotide products on endonuclease-generated singly-nicked circular duplex DNA may represent an important biological function or signal in lymphoid cells.