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 VB 7.1 element is associated with CB1 (99 clones) or CB2 (90 clones) isot ypes of the T-cell receptor (TCR) P chain in the Mexican axolotl. Thre e stages of development were analyzed: 2.5 months, 10 months, and 25 m onths. Three JBI segments were associated with the VB-CBI rearrangemen ts and six JB2 segments with VB-CB2. As in other vertebrates, some ami no acid positions were conserved in all J beta s (e.g., Phe-108, Gly-1 09, Gly-111, Thr-112, and Val-116). Two 11 nucleotides DB-like sequenc es, 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 se gments 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 organize d 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-ol d larvae had N nucleotides, compared with about 73% in 10-25-month old animals. The beta-chain VDJ junctions had about 30% of defective rear rangements at all stages of development, which could be due to the slo w rate of cell division in the axolotl lymphoid organs, and the large genome in this urodele. Many of the axolotl CDR beta 3 sequences deduc ed for in frame VDJ rearrangements are the same in animals of differen t origins. Such redundancy could be a statistical effect due to the sm all number of thymocytes in the developing axolotl, rather than to som e bias due to junctional preferences.