The structure, rearrangement and ontogenic expression of DB and JB gene segments of the Mexican axolotl T-cell antigen receptor beta chain (TCRB)

We sequenced a total of 189 independent rearrangements in which the VB7.1 element is associated with CB1 (99 clones) or CB2 (90 clones) isotypes of the T-cell receptor (TCR) P chain in the Mexican axolotl. Three stages of development were analyzed: 2.5 months, 10 months, and 25 months. Three JBI segments were associated with the VB-CBI rearrangements and six JB2 segments with VB-CB2. As in other vertebrates, some amino acid positions were conserved in all J beta s (e.g., Phe-108, Gly-109, Gly-111, Thr-112, and Val-116). Two 11 nucleotides DB-like sequences, differed by one (A or T) central residue and could be productively read in the three putative reading frames. Most of the DB1 and JB1 segments were in the VB-CB1 clones, and most of the DB2 and JB2 segments were in the VB-CB2 clones, suggesting that the TCRB locus is organized into independent DB-JB-CB clusters that used the same collection of VB segments. About 40% of the beta-chain VDJ junctions in 2.5-month-old larvae had N nucleotides, compared with about 73% in 10-25-month old animals. The beta-chain VDJ junctions had about 30% of defective rearrangements at all stages of development, which could be due to the slow rate of cell division in the axolotl lymphoid organs, and the large genome in this urodele. Many of the axolotl CDR beta 3 sequences deduced for in frame VDJ rearrangements are the same in animals of different origins. Such redundancy could be a statistical effect due to the small number of thymocytes in the developing axolotl, rather than to some bias due to junctional preferences.