Terminal deoxynucleotidyl transferases from elasmobranchs reveal structural conservation within vertebrates.

The DNA polymerase (pol) X family is an ancient group of enzymes that function in DNA replication and repair (pol β), translesion synthesis (pol λ and pol μ) and terminal addition of non-templated nucleotides. This latter terminal deoxynucleotidyl transferase (TdT) activity performs the unique function of providing diversity at coding joins of immunoglobulin and T-cell receptor genes. The first isolated full-length TdT genes from shark and skate are reported here. Comparisons with the three-dimensional structure of mouse TdT indicate structural similarity with elasmobranch orthologues that supports both a template-independent mode of replication and a lack of strong nucleotide bias. The vertebrate TdTs appear more closely related to pol μ and fungal polymerases than to pol λ and pol β. Thus, unlike other molecules of adaptive immunity, TdT is a member of an ancient gene family with a clear gene phylogeny and a high degree of similarity, which implies the existence of TdT ancestors in jawless fishes and invertebrates. [ABSTRACT FROM AUTHOR]