Terminal Deoxynucleotidyl Transferase (TdT) Terminal Deoxynucleotidy transferase (TdT) is a template independent polymerase that catalyzes the addition of deoxyribonucleotide monophosphate from triphosphate to the 3' hydroxyl terminus of DNA. Description:• Prefered substrates are: single-stranded DNA, double-stranded DNA with 3’-hydroxyl termini and oligodeoxynucleotide primers (1).• Used for specific labeling of 3'-termini with ribonucleotides (2).• Labels 3'-ends of DNA fragments with an [a-32P] 3'-deoxynucleoside (3).• Adds homopolymer tails of deoxyribonucleotides to vectors or cDNAs (4,5).Unit Definition: One unit is defined as the amount of enzyme required to transfer 1 nmol of dAMP from dATP to the 3'-OH terminus of the oligodeoxyribonucleotide initiator p(dA)50 in 1 hr at 37°C. Storage Conditions: Store at -20°C. Storage Buffer: 100 mM Tris-HCI (pH 7.2 at 22°C), 1 mM dithiothreitol and 50% (v/v) glycerol. Assay Conditions: 40 mM potassium cacodylate (pH 7.2), 8 mM MgCl2, 8.3 mM potassium phosphate, 0.33 mM ZnSO4, 10 mg/ml bovine serum albumin, 0.01 mM oligodeoxynucleotide p(dA)50 and 1 mM [a-³²P]dATP in 1 hr at 37oC in a reaction volume of 60 µl. Quality Control: All preparations are tested for endonuclease, exonuclease and nonspecific RNase and single- and double-stranded DNase activities. References:1. Chang, L.M. and Bollum, F.J. (1971) J. Biol. Chem. 246, 909-916.2. Roychoudhury, R. and Kossel, H. (1971) Eur. J. Biochem. 22, 310-320.3. Tu, C.P.D., and Cohen, S.N. (1980) Gene 10, 177-1834. Roychoudhury, R., Jay, E. and Wu, R. (1976) Nucleic Acids Res. 3, 863-877.5. Deng, G. and Wu, R. (1983) Methods Enzymol. 100, 96-116.