Specialized DNA Polymerases in Lymphoid Cells

Excerpt

During early attempts to isolate DNA polymerases from mammalian cells, it was noted that a major polymerase activity recoverable from calf thymus tissue was one unable to copy a template. Rather it formed a DNA polymer whose composition was determined by the type of deoxyribonucleoside triphosphates provided in the reaction mixture (Bollum 1960; Krakow et al. 1962; Bollum 1974). This enzyme was named “terminal deoxynucleotidyl transferase.” In retrospect, it is clear that the discovery of terminal transferase was an accident of the choice of thymus as starting material. This choice was presumably made because thymocytes have a very high ratio of nucleus to cytoplasm and a very rapid rate of DNA synthesis.

Terminal transferase attracted little attention from biologists until its unique tissue distribution was appreciated: under normal conditions, it is only present in thymus and bone marrow (Chang 1971; McCaffrey et al. 1973, 1975; Coleman et al. 1974; Kung...