Terminal deoxynucleotidyl transferase

Terminal deoxynucleotidyl transferase (TdT), also known as DNA nucleotidylexotransferase (DNTT) or terminal transferase, is a specialized is_associated_with::DNA polymerase expressed in immature, pre-B, pre-T is_associated_with::lymphoid cells, and is_associated_with::acute lymphoblastic leukemia/lymphoma cells. TdT adds is_associated_with::N-nucleotides to the V,D, and J is_associated_with::exons during is_associated_with::antibody gene recombination, enabling the phenomenon of is_associated_with::junctional diversity. In humans, terminal transferase is encoded by the DNTT is_associated_with::gene.

TdT is absent in fetal liver HSCs, significantly impairing is_associated_with::junctional diversity in B-cells during the fetal period.

Function
TdT catalyses the addition of is_associated_with::nucleotides to the 3' terminus of a is_associated_with::DNA molecule. Unlike most DNA polymerases, it does not require a template. The preferred substrate of this enzyme is a 3'-overhang, but it can also add nucleotides to blunt or recessed 3' ends. is_associated_with::Cobalt is a necessary cofactor, however the enzyme catalyzes reaction upon Mg and Mn administration in vitro.

Uses
Terminal transferase has applications in is_associated_with::molecular biology. It can be used in RACE to add nucleotides that can then be used as a template for a primer in subsequent is_associated_with::PCR. It can also be used to add nucleotides labeled with radioactive is_associated_with::isotopes, for example in the is_associated_with::TUNEL assay (Terminal deoxynucleotidyl transferase dUTP Nick End Labeling) for the demonstration of is_associated_with::apoptosis (which is marked, in part, by fragmented DNA). Also used in the immunofluorescence assay for the diagnosis of is_associated_with::acute lymphoblastic leukemia.

In is_associated_with::immunohistochemistry, antibodies to TdT can be used to demonstrate the presence of immature T and B cells and multipotent haematopoietic stem cells, which possess the antigen, while mature lymphoid cells are always TdT-negative. While TdT-positive cells are found in small numbers in healthy lymph nodes and tonsils, the malignant cells of acute lymphoblastic leukaemia are also TdT-positive, and the antibody can, therefore, be used as part of a panel to diagnose this disease and to distinguish it from, for example, small cell tumours of childhood.