Molecular biology of terminal transferase

Terminal transferase is an unusual deoxynucleotide polymerizing enzyme found only in prelymphocytes. The protein was purified to homogeneity from calf thymus glands in 1971 as a 32 kDa protein with a two peptide structure. Subsequent biochemical and immunological analyses of terminal transferase protein in crude extracts from a number of animal species showed a single peptide with a molecular weight of about 58,000. The two peptide structure found earlier was caused by proteolysis. Homogeneous 58 kDa terminal transferase has now been produced from human lymphoblastoid cells and calf thymus glands by immunoaffinity chromatography. In vitro phosphorylation studies showed that the terminal transferase protein contains one phosphorylation site near one end of the polypeptide chain, and the phosphorylation of the enzyme has been confirmed by in vivo labeling experiments. Unambiguous demonstration of the molecular weight of the human terminal transferase was obtained by translation of the cloned human terminal transferase DNA sequence to a 58,308 Da protein. The translated amino acid sequence also provided a possible phosphorylation site near the amino-terminus of the protein. Preliminary analysis of the genomic structure shows a simple intron/exon pattern with the total human terminal transferase gene spanning at least 65 Kb.