CHAPTER 7
Terminal Deoxyribonucleotidyl
Transferase (EC 2.7.7.31)Frank Grosse and Andreas Manns
- Introduction
1.1. The Reaction
Terminal deoxyribonucleotidyl transferase (TdT) (EC 2.7.7.31) cata-
lyzes the addition of deoxynucleoside triphosphates to the 3' ends of
oligo- and polynucleotide primers. Different from all the other known
DNA polymerases, TdT-catalyzed DNA synthesis is not template-
directed. The reaction can be described as:
n dNTP + d(pX)m M~ Td----~> ÷ d(pX)m(pN)n + nPPi (1)Only one dNTP is necessary for polymerization, although any dNTP
(and derivatives thereof) or any combination of dNTPs will serve as
substrate.
1.2. Biological Function of TdT
TdT activity is only found in the nuclei of pre-T and pre-B lympho-
cytes during immunopoiesis. Therefore, it has been proposed that this
enzyme might play a role in creating somatic diversity of immunoglo-
bulin and T-cell receptor genes (1). This hypothesis has gained indirect
support by the demonstration that the existence of N regions, i.e.,
small numbers of nucleotides that are not encoded by the chromo-
somes, correlates with the expression of TdT. Besides this circumstan-
tial evidence for a role as an active mutator, after 25 years of intensive
From: Methods in Molecular Biology, Vol. 16: Enzymes of Molecular Biology
Edited by: M. M. Burrell Copyright ©1993 Humana Press Inc., Totowa, NJ95