EVOLUTION OF T-CELL RECEPTOR GENES - EXTENSIVE DIVERSITY OF V-BETA FAMILIES IN THE MEXICAN AXOLOTL

We have cloned 36 different rearranged variable regions (V beta) genes encoding the beta-chain of the T cell receptor in an amphibian specie s, Ambystoma mexicanum (the Mexican axolotl). Eleven different V beta segments were identified, which can be classified into 9 families on t he basis of a minimum of 75% nucleotide identity. All the cloned V bet a segments have the canonical features of known mammalian and avian V beta, including conserved residues Cys(23), Trp(34), Arg(69), Tyr(90), and Cys(92). There seems to be a greater genetic distance between the axolotl V beta families than between the different V beta families of any mammalian species examined to date: most of the axolotl V beta s have fewer than 35% identical nucleotides and the less related familie s (V beta 4 and V beta 8) have no more than 23.2% identity (13.5% at t he amino acid level). Despite their great mutual divergence, several a xolotl V beta are sequence-related to some mammalian V beta genes, lik e the human V beta 13 and V beta 20 segments and their murine V beta 8 and V beta 14 homologues. However, the axolotl V beta 8 and V beta 9 families are not significantly related to any other V beta sequence at the nucleotide level and show limited amino acid similarity to mammal ian V alpha, V kappa III, or V-H sequences. The detection of nine V be ta families among 35 randomly cloned V beta segments suggests that the V beta gene repertoire in the axolotl is probably larger than present ly estimated.