Isolation and characterization of the Xenopus terminal deoxynucleotidyl transferase

A cDNA clone containing the sequence of the Xenopus homologue of terminal deoxynucleotidyl transferase (TdT) was isolated and analyzed. The derived amino acid sequence shares > 50% identity with the human, mouse, and bovine TdT sequences. By hybridization and PCR analyses Xenopus TdT mRNA was found in the thymus and not in the spleen, kidney, intestine, or liver. During ontogeny TdT appears in significant amounts in the thymus of tadpoles at metamorphic climax but little in the earlier midlarval stages. The emergence of TdT only late in development correlates with the paucity of N region addition in larval Ig heavy chain sequences. Past experiments on the ontogeny of the Xenopus immune system revealed a less efficacious tadpole immune response in skin graft rejection and Ab heterogeneity; the absence of TdT in the tadpole stages fits well with the notion of lower larval TCR and Ab diversity.