Terminal deoxynucleotidyl transferase (TdT), also known as DNA nucleotidylexotransferase (DNTT) or terminal transferase, is a specialized DNA polymerase expressed in immature, pre-B, pre-T lymphoid cells, and acute lymphoblastic leukemia/lymphoma cells. TdT adds N-nucleotides to the V, D, and Jexons of the TCR and BCR genes during antibody gene recombination, enabling the phenomenon of junctional diversity. In humans, terminal transferase is encoded by the DNTTgene.[5][6] As a member of the X family of DNA polymerase enzymes, it works in conjunction with polymerase λ and polymerase μ, both of which belong to the same X family of polymerase enzymes. The diversity introduced by TdT has played an important role in the evolution of the vertebrate immune system, significantly increasing the variety of antigen receptors that a cell is equipped with to fight pathogens. Studies using TdT knockout mice have found drastic reductions (10-fold) in T-cell receptor (TCR) diversity compared with that of normal, or wild-type, systems. The greater diversity of TCRs that an organism is equipped with leads to greater resistance to infection.[7][8] Although TdT was one of the first DNA polymerases identified in mammals in 1960,[9] it remains one of the least understood of all DNA polymerases.[7] In 2016–18, TdT was discovered to demonstrate in trans template dependant behaviour in addition to its more broadly known template independent behaviour[10][11]
TdT is absent in fetal liver HSCs, significantly impairing junctional diversity in B-cells during the fetal period.[12]
^Yang-Feng TL, Landau NR, Baltimore D, Francke U (1986). "The terminal deoxynucleotidyltransferase gene is located on human chromosome 10 (10q23----q24) and on mouse chromosome 19". Cytogenetics and Cell Genetics. 43 (3–4): 121–6. doi:10.1159/000132309. PMID3467897.
^Hardy R (2008). "Chapter 7: B Lymphocyte Development and Biology". In Paul W (ed.). Fundamental Immunology (Book) (6th ed.). Philadelphia: Lippincott Williams & Wilkins. pp. 237–269. ISBN978-0-7817-6519-0.
