Structure and diversity of T-lymphocyte antigen receptors alpha and gamma in Xenopus

Short primer PCR directed at conserved regions of amino acid sequence within the T-cell antigen receptor (TCR) and immunoglobulin (Ig) light chain variable (V) regions was used to amplify putative TCRγ V region amplicons from stage 45 Xenopus laevis (African clawed frog) mRNA (cDNA). An adult Xenopus spleen cDNA library was screened using the Vγ and a putative TCRVα amplicon. Full copy length cDNAs containing the specific PCR-derived Vα and Vγ amplicons were recovered at relatively low frequency. Probes complementing the TCRα and TCRγ constant (C) regions were employed to isolate equivalent numbers of additional TCR α and TCR γ cDNAs in an unbiased (non-V-based) manner. Few Vγ genes appear to be expressed relative to the highly diverse expression of V α genes in equivalent numbers of cDNAs that were analyzed. Two TCRγ C regions differ at only two positions; whereas two TCRα C regions differ at 33 coding positions as well as in their respective 3′ untranslated regions, consistent with two independent loci. However, genomic Southern blots revealed considerably higher numbers of hybridizing bands when probed with C γ than with C α. A potential novel mechanism of diversification is suggested by an unusual TCR α cDNA in which the V region can be translated in two frames through utilization of two closely linked V genes and an alternative splicing process. This process produces a translatable cDNA that is not readily predictable from the genomic locus utilizing normal recombination and splicing mechanisms.