An amphibian CD3 homologue of the mammalian CD3 γ and δ genes

T cell receptor (TCR) genes have been identified in representatives of both cartilaginous and bony vertebrates. The CD3 chains that serve as signal transducing elements of the TCR complex in mammals have been defined to a limited extent in birds. In these studies a CD3 homologue was identified in an amphibian representative, Xenopus laevis, using degenerate oligomer primers designed from conserved regions of avian and mammalian CD3γ/δ subunits. The reverse transcriptase polymerase chain reaction amplified product of Xenopus splenocyte RNA was then used to isolate full‐length cDNA clones from a splenic library. When employed as probes, the cDNA clones hybridized with a 1‐kb mRNA transcript in Xenopus T cells, but not in other cell types. Comparison of the deduced amino acid sequence indicated a similar degree of homology with mammalian and avian CD3 γ and δ chains. Genomic analysis indicated that the Xenopus CD3 molecule is encoded by five exons, a structure resembling the mammalian CD3δ gene rather than the seven exon CD3γ gene. Southern blot analysis and sequencing of the 5′ flanking region failed to yield evidence of a related Xenopus gene. This amphibian CD3 gene thus appears to represent an ancestral form of the mammalian CD3 γ and δ genes.