BARTL, S.: Terminal Deoxynucleotidyl Transferase (TdT) Genes Isolated from Cartilaginous Fishes

Terminal Deoxynucleotidyl Transferase (TdT) is a nuclear enzyme that adds nontemplated nucleotides to free 3’-hydroxyl ends of DNA fragments. In mammals, the only documented function for TdT is the addition of nucleotides at gene element (V, D, and J) junctions during immunoglobulin (Ig) and T cell receptor (TCR) gene rearrangement thus diversifying the receptor repertoire. Since TdT is expressed in pre-lymphocytes in the early stages of differentiation it serves as a marker for primary lymphoid tissues. Here the isolation of TdT genes from cartilaginous fishes is reported. These genes serve as useful markers of immune function and provide insight into the evolution of adaptive immunity. They will be discussed in the context of other genes of immunological importance previously found in sharks and rays, in particular Ig, TCR, Major Histocompatibility Complex genes (required for antigen recognition by T cells) and recombination activating genes (essential for gene rearrangement). The evolution of the DNA polymerase beta gene family, to which TdT genes belong, also will be discussed.